This adds a googletest-based test harness (with googletest pulled into
`testing/googletest`, so we have a fixed state of it), and a few test
cases that demonstrate its use.
Original work by Eric Paniagua in #898, with minor cleanups by Gergely
Nagy.
Signed-off-by: Eric Paniagua <epaniagua@google.com>
Signed-off-by: Gergely Nagy <algernon@keyboard.io>