Update short secret raise tests

[Synse]

This PR updates the test_encrypt_raises_value_error_short_secret and test_decrypt_raises_value_error_short_secret1 tests to match the raised ValueError string.

This should prevent cases where something is raising but in the way it's expected to.

🔍 References

• cc Pad short secrets #14 (comment)
• cc https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRaisesRegex