Test code in C should use test-specific assertions that throw errors
instead of exiting the program with a signal (crash). Not only does
this provide more useful and easily accessible information of the
failing condition, but it also allows running the test suite without
assertions (`USE_ASSERT_CHECKING`) enabled. Having assertions disabled
during tests also provides more accurate test coverage numbers. Note
that these test-specific assertions are not intended to replace
regular assertions (`Assert`), which are used in non-test code.
The way to enable (or disable) assertions in CMake has also been
simplified and cleaned up. The option `-DASSERTIONS=[ON|OFF]` can be
used to enable assertions for a build, unless already enabled in the
PostgreSQL one is building against (in which case that setting takes
precedence). Note that the `ASSERTIONS` option defaults to `OFF` since
it is no longer necessary to have assertions enabled for tests.
