Modify snapshot_util to check and create snapshot if not present already for Accessibility tests

[ajitava-git]

Running Accessibility Tests involves two steps:

1. Use --snapshot-update param with pytest when running the accessibility test to create snapshots in tests/snapshot/<accessibility_test_name>/<test_function> dir.
2. Run the test and pytest validates the accessibility features against the snapshots in tests/snapshot/<accessibility_test_name>/<test_function> dir

step 1 can be skipped if the snapshots are already present in tests/snapshot/<accessibility_test_name>/<test_function>

Modify the snapshot_util module to check if snapshots are available in tests/snapshot/<accessibility_test_name>/<test_function>, if the snapshots are not present then create the snapshots & skip running the tests.