selftests: vdso: Rename vdso_test to vdso_test_gettimeofday

Currently the vDSO kselftests have a test called vdso_test which tests
the vDSO implementation of gettimeofday(). In preparation for adding
tests for other vDSO functionality rename this test to reflect what's
going on.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>