Fix initial Java migration

This commit fixes several issues with the initial java migration.

On MacOS, the name of the temp directory can be `T` and the new tgz API
requires a parent folder whose name has at least 3 characters.

The Selenium tests require a more precise element to show the advanced
section.

The integration tests were generation the docs snippet on the wrong
accept header.

initializr-generator/src/main/groovy/io/spring/initializr/generator/ProjectGenerator.groovy

@@ -67,9 +67,10 @@ class ProjectGenerator {
 	@Autowired
 	ProjectResourceLocator projectResourceLocator = new ProjectResourceLocator()
 
-	@Value('${TMPDIR:.}')
+	@Value('${TMPDIR:.}/initializr')
 	String tmpdir
 
+	private File temporaryDirectory
 	private transient Map<String, List<File>> temporaryFiles = [:]
 
 	/**
@@ -126,7 +127,7 @@ class ProjectGenerator {
 	protected File doGenerateProjectStructure(ProjectRequest request) {
 		def model = resolveModel(request)
 
-		def rootDir = File.createTempFile('tmp', '', new File(tmpdir))
+		def rootDir = File.createTempFile('tmp', '', getTemporaryDirectory())
 		addTempFile(rootDir.name, rootDir)
 		rootDir.delete()
 		rootDir.mkdirs()
@@ -182,11 +183,19 @@ class ProjectGenerator {
 	 * directory and extension
 	 */
 	File createDistributionFile(File dir, String extension) {
-		def download = new File(tmpdir, dir.name + extension)
+		def download = new File(getTemporaryDirectory(), dir.name + extension)
 		addTempFile(dir.name, download)
 		download
 	}
 
+	private File getTemporaryDirectory() {
+		if (!temporaryDirectory) {
+			temporaryDirectory = new File(tmpdir, 'initializr')
+			temporaryDirectory.mkdirs()
+		}
+		temporaryDirectory
+	}
+
 	/**
 	 * Clean all the temporary files that are related to this root
 	 * directory.