validation: more test cases for masked paths tests
validation: more test cases for masked paths tests

Split out the test function into `checkMaskedPaths()`, and add more
cases for masked paths like subdirectory, file under subdirectory,
and directory under subdirectory.

Signed-off-by: Dongsu Park <696fb7639d678a1e692fa501217d7509545d7331@kinvolk.io>

opencontainers/ocitools,opencontainers/runtime-tools,opencontainers/ocitools,opencontainers/runtime-tools