Move devcontainer_test.go to top level

devcontainer_test imports the top-level envbuilder package, which
creates a transitive dependency on kaniko for external code that imports
envbuilder/devcontainer. Moving the test file to the top level avoids
this dependency.

devcontainer/devcontainer_test.go → devcontainer_test.go

@@ -1,4 +1,4 @@
-package devcontainer_test
+package envbuilder_test
 
 import (
 	"crypto/md5"