Add another (CheckResult && CheckResult) operator.

It’s useful to be able to combine a CheckResult R-value with a CheckResult (non-const) L-value.
Provide access to a CheckResult’s message.
Pass message(s) to combined CheckResult.
Improved vector formatting.

Make a vector look more like C++.
Use symbolic name for segmentation faults.
Add support for individual test timeouts.

Previously, we had to time out the entire process with, e.g., gtimeout. Catching errors like this at the test granularity is kind of the point of this library, however, so let’s do it.
Add support for named test executions.
Use fewer compiler warnings.

Stick to warnings that both Clang and GCC support.
Add debug symbols to debug builds!
Trivial: whitespace cleanup.