tests/fsck: add malleable_base64 test filesystem

This filesystem contains filenames with non-canonical base64
encodings of the same name "foo", leading to this mess:

$ ls mnt/
foo  foo  foo  foo

1 files changed, 11 insertions, 0 deletions

diff --git a/tests/fsck/fsck_test.go b/tests/fsck/fsck_test.go
index cebc86b..b70fd49 100644
--- a/tests/fsck/fsck_test.go
+++ b/tests/fsck/fsck_test.go
@@ -49,6 +49,17 @@ func TestBrokenFsV14(t *testing.T) {
 	}
 }
 
+func TestMalleableBase64(t *testing.T) {
+	cmd := exec.Command(test_helpers.GocryptfsBinary, "-fsck", "-extpass", "echo test", "malleable_base64")
+	outBin, err := cmd.CombinedOutput()
+	out := string(outBin)
+	t.Log(out)
+	code := test_helpers.ExtractCmdExitCode(err)
+	if code != exitcodes.FsckErrors {
+		t.Errorf("wrong exit code, have=%d want=%d", code, exitcodes.FsckErrors)
+	}
+}
+
 func TestExampleFses(t *testing.T) {
 	dirfd, err := os.Open("../example_filesystems")
 	if err != nil {