This allows you to write EXPECT_OK(statusor), as well as
EXPECT_THAT(status, IsStatusCode(StatusCode::kInvalidArgument).
Change-Id: I53bed694d812c501eb305ed4ddb358e1f9a68277
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1704
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>