tests: ensure failure if input file doesn't exist

tests: ensure failure if input file doesn't exist

When using the DO_TEST_PARSE_ERROR() macro, a failure to parse the input
file is considered a successful test. However, if the input file is
totally missing, that should be distinguished from a parsing error and
not be treated as a test success.

The function virDomainDefParseFile() simply returns NULL for any parse
failure, including a missing file. So we need to explicitly check
whether the file exists first, and fail the test if it is missing.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>