Fix a flaky test.

[LALAYANG]

1. The test failure:

• I ran mvn edu.illinois:nondex-maven-plugin:1.1.2:nondex -Dtest=com.lazerycode.jmeter.json.TestConfigTest#createConfigFromResourceFile
  Here are the details of NonDex.
• The test com.lazerycode.jmeter.json.TestConfigTest#createConfigFromResourceFile fails as follows:

at com.lazerycode.jmeter.json.TestConfigTest.createConfigFromResourceFile(TestConfigTest.java:28)
Error: TestConfigTest.createConfigFromResourceFile:28 
expected:<"{"[executionID":"test-execution","jmeterDirectoryPath":null,"runtimeJarName":null,"resultsOutputIsCSVFormat":false,"generateReports":false,"resultFilesLocations":[]],"propertiesMap":nul...> 
but was:<"{"[generateReports":false,"resultsOutputIsCSVFormat":false,"jmeterDirectoryPath":null,"resultFilesLocations":[],"executionID":"test-execution","runtimeJarName":null],"propertiesMap":nul...>

2. Why it fails:

• In src/main/java/com/lazerycode/jmeter/json/TestConfigurationWrapper.java, mapper.writeValueAsString(testConfiguration) serializes Java objects as JSON output, but the order of objects being mapped into a string by ObjectMapper changes.

3. Fix it:

• Here we change assertThat to assertThatJson, also need to add a dependency for net.javacrumbs.json-unit