iat-verifier: Use direct imports for tests modules

The following files have been changed from using:
`from tests.<module name> import ...` to
`from <module name> import ...`

Files changed:
- synthetic_token_verifier.py
- test_synthetic.py

Modules it was changed for:
- synthetic_token_claims
- synthetic_token_verifier
- test_utils

Allowing `nose2` to discover and run the tests without the `tests`
directory needing to be a python package.

Change-Id: Icb717dd676944ab6c71f9a35218718e37c625a1f
Signed-off-by: Matt Dalzell <matt.dalzell@arm.com>
2 files changed