Fix allowlist for unreliable pinning.

When checking IsUnreliablePinningAllowed, the existing code calls
IsTestDir on a value starts with "/data", but IsTestDir expects
metadata_dir_. Fix it.

Also, make subdir checks more robust and add test cases for IsSubdir.

Fixes: 162557082
Test: vts_libsnapshot_test
Test: fiemap_image_test

Change-Id: I495cfae3da11d1e0800b8abf520df10dd9a29dce