Move test_pemutils.py to unit test directory

The test_pemutils.py module consists of unit tests for common Keystone
code. This commit moves the test_pemutils.py unit tests to the
keystone/tests/unit/common directory.

Change-Id: Ifeedaaf2fb7cd379bb9e872567874bced0c94b02
Merge "Move test_pemutils.py to unit test directory"