snapuserd_test: check writer != nullptr

Cow Writer used to fail with ENOSPC during initialization.

Try temporaryFile creation with path set to current working directory.

Bug: 328879200
Test: th, snapuserd_test
Change-Id: I7b3833967952c74142f1d5a35cb8d94dd6d894fc
Signed-off-by: Akilesh Kailash <akailash@google.com>