It's very annoying having to remember the right incant every time I want
to switch around between my build, build-release, build-asan, etc.,
output directories.

Unfortunately, this target is pretty unfriendly without CMake 3.2+ (and
Ninja 1.5+). This combination gives a USES_TERMINAL flag to
add_custom_target which uses Ninja's "console" pool, otherwise the
output buffering gets in the way. Ubuntu LTS is still on an older CMake,
so do a version check in the meantime.

CMake also has its own test mechanism (CTest), but this doesn't use it.
It seems to prefer knowing what all the tests are and then tries to do
its own output management and parallelizing and such. We already have
our own runners. all_tests.go could actually be converted tidily, but
generate_build_files.py also needs to read it, and runner.go has very
specific needs.

Naming the target ninja -C build test would be nice, but CTest squats
that name and CMake grumps when you use a reserved name, so I've gone
with run_tests.

Change-Id: Ibd20ebd50febe1b4e91bb19921f3bbbd9fbcf66c
Reviewed-on: https://boringssl-review.googlesource.com/6270
Reviewed-by: Adam Langley <alangley@gmail.com>