[libc++] Remove signal-based checkpoints in libc++ tests

While this adds some convenience to the test suite, it prevents the tests
using these checkpoints from being used on systems where signals are not
available, such as some embedded systems. It will also prevent these tests
from being constexpr-friendly once e.g. std::map is made constexpr, due
to the use of statics.

Instead, one can always use a debugger to figure out exactly where a
test is failing when that isn't clear from the log output without
checkpoints.