Rename requires files to standard names.

Rename tools/pip-requires to requirements.txt and tools/test-requires
to test-requirements.txt. These are standard files, and tools in the
general world are growing intelligence about them.

Change-Id: Ib3e50a811868e2969923d978ee00c4f92682aa1c
Fixes: bug #1179008
Merge "Rename requires files to standard names."