faultrep/tests: Update is_process_limited() to deal with non-administrator, non-power user accounts.

Unfortunately I was only able to test this by overriding the GL_RENDERER and
GL_VENDOR strings returned by the driver.

Note that we could have enough privileges to do the InstallProduct
tests, but not enough to clean up the registry after them, thus
causing later runs to fail. In that case we skip the tests.