Allow recursive test directories

[skuzzle]

The @FilesFrom annotation already has a flag to enable recursive scanning for test input. We could support the same for @DirectoriesFrom.
When recursively iterating directories, we need a way to determine which directory will be actually used as test input and which directory just represents an intermediate level for structuring the tests.

Suggested API:

This test will only use leave directories (those that don't contain any other directories) as input:

    @ParameterizedTest
    @DirectoriesFrom(testResourcesDirectory = "test-directories-recursive", recursive = true)
    void testDirectoriesRecursivelyDefaultFilter(TestDirectory directory, Snapshot snapshot) throws IOException {
        // ...
    }

This test uses a TestDirectoryFilter for selecting the directories that are used as actual test input

    @DirectoriesFrom(testResourcesDirectory = "test-directories-recursive", recursive = true,
            filter = SampleTestDirectoryFilter.class)
    void testDirectoriesRecursivelyCustomFilter(TestDirectory directory, Snapshot snapshot) throws IOException {
        // ...
    }

    static class SampleTestDirectoryFilter implements TestDirectoryFilter {

        @Override
        public boolean include(TestDirectory testDirectory, boolean recursive) throws IOException {
            return testDirectory.containsFile("input.txt");
        }

    }