Support "with"-style errno assertions.

This adds the option to use assertRaisesErrno as a context manager, so
blocks of code can be tested instead of just callable objects. It is
also a more readable style when the callable takes many args.

Test: all_tests.sh passing on device kernels
Change-Id: Ib6fd88c3a1a4fd1b69403b97712c7f27732ade3a