Refactor MkdirStepRunner and test it (#92)

`MkdirStepRunner` will now:
- handle both relative and absolute paths
- throw an exception if the directory it wants to create already exists
- throw an original Symfony Filesystem `IOException` not rewrapping it
into a `BlueprintException`, as agreed in #65

### What problem does it fix?
- until now the runner would only handle absolute paths
- has thrown an exception in a bad way
- we use this step in the `examples` directory, and since it only
accepted absolute paths and we have provided it a relative path the
example was flawed

### How to test if it works?
- this PR includes a full suite of tests for this runner

src/WordPress/Blueprints/Runner/Step/MkdirStepRunner.php

@@ -2,19 +2,22 @@
 
 namespace WordPress\Blueprints\Runner\Step;
 
+use Symfony\Component\Filesystem\Filesystem;
+use WordPress\Blueprints\BlueprintException;
 use WordPress\Blueprints\Model\DataClass\MkdirStep;
 
 
 class MkdirStepRunner extends BaseStepRunner {
 
 	/**
-	 * @param \WordPress\Blueprints\Model\DataClass\MkdirStep $input
+	 * @param MkdirStep $input
 	 */
-	function run( $input ) {
-		// @TODO: Treat $input->path as relative path to the document root (unless it's absolute)
-		$success = mkdir( $input->path );
-		if ( ! $success ) {
-			throw new \Exception( "Failed to create directory at {$input->path}" );
+	function run( MkdirStep $input ) {
+		$resolved_path = $this->getRuntime()->resolvePath( $input->path );
+		$filesystem   = new Filesystem();
+		if ( $filesystem->exists( $resolved_path ) ) {
+			throw new BlueprintException( "Failed to create \"$resolved_path\": the directory exists." );
 		}
+		$filesystem->mkdir( $resolved_path );
 	}
 }

tests/unit/steps/MkdirStepRunnerTest.php

@@ -0,0 +1,113 @@
+<?php
+
+namespace unit\steps;
+
+use PHPUnitTestCase;
+use Symfony\Component\Filesystem\Filesystem;
+use Symfony\Component\Filesystem\Path;
+use WordPress\Blueprints\Model\DataClass\MkdirStep;
+use WordPress\Blueprints\Runner\Step\MkdirStepRunner;
+use WordPress\Blueprints\Runtime\Runtime;
+use WordPress\Blueprints\BlueprintException;
+
+class MkdirStepRunnerTest extends PHPUnitTestCase {
+	/**
+	 * @var string
+	 */
+	private $document_root;
+
+	/**
+	 * @var Runtime
+	 */
+	private $runtime;
+
+	/**
+	 * @var MkdirStepRunner
+	 */
+	private $step_runner;
+
+	/**
+	 * @var Filesystem
+	 */
+	private $filesystem;
+
+	/**
+	 * @before
+	 */
+	public function before() {
+		$this->document_root = Path::makeAbsolute( "test", sys_get_temp_dir() );
+		$this->runtime = new Runtime( $this->document_root );
+
+		$this->step_runner = new MkdirStepRunner();
+		$this->step_runner->setRuntime( $this->runtime );
+
+		$this->filesystem = new Filesystem();
+	}
+
+	/**
+	 * @after
+	 */
+	public function after() {
+		$this->filesystem->remove( $this->document_root );
+	}
+
+	public function testCreateDirectoryWhenUsingRelativePath() {
+		$path = 'dir';
+		$step = new MkdirStep();
+		$step->setPath( $path );
+
+		$this->step_runner->run( $step );
+
+		$resolved_path = $this->runtime->resolvePath( $path );
+		self::assertDirectoryExists( $resolved_path );
+	}
+
+	public function testCreateDirectoryWhenUsingAbsolutePath() {
+		$relative_path = 'dir';
+		$absolute_path = $this->runtime->resolvePath( $relative_path );
+
+		$step = new MkdirStep();
+		$step->setPath( $absolute_path );
+
+		$this->step_runner->run( $step );
+
+		self::assertDirectoryExists( $absolute_path );
+	}
+
+	public function testCreateDirectoryRecursively() {
+		$path = 'dir/subdir';
+		$step = new MkdirStep();
+		$step->setPath( $path );
+
+		$this->step_runner->run( $step );
+
+		$resolved_path = $this->runtime->resolvePath( $path );
+		self::assertDirectoryExists( $resolved_path );
+	}
+
+	public function testCreateReadableAndWritableDirectory() {
+		$path = 'dir';
+		$step = new MkdirStep();
+		$step->setPath( $path );
+
+		$this->step_runner->run( $step );
+
+		$resolved_path = $this->runtime->resolvePath( $path );
+		self::assertDirectoryExists( $resolved_path );
+		self::assertDirectoryIsWritable( $resolved_path );
+		self::assertDirectoryIsReadable( $resolved_path );
+	}
+
+	public function testThrowExceptionWhenCreatingDirectoryAndItAlreadyExists() {
+		$path = 'dir';
+		$resolved_path = $this->runtime->resolvePath( $path );
+		$this->filesystem->mkdir( $resolved_path );
+
+		$step = new MkdirStep();
+		$step->setPath( $path );
+
+		self::expectException( BlueprintException::class );
+		self::expectExceptionMessage( "Failed to create \"$resolved_path\": the directory exists." );
+		$this->step_runner->run( $step );
+	}
+}