oeqa/selftest: rename git.py to intercept.py

By naming this test class git.py, any attempt to import GitPython (as
needed by oelib.buildhistory) failed.

As this class exercises the intercepts, rename it to intercept.py.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>