When raising an error we print the current file, line and function where
the error was raised. This is of course quite helpful to users, but it
stands in the way of testing the clar because they make the output of
executables dependent on the developer's setup.

Add the ability to override these with fixed constants via a new
`CLAR_SELFTEST` macro.

We use `exit(-1)` in a lot of places. Exit codes should be in the range
of [0, 255] though, so the resulting behaviour is not defined and
depends on the platform. Fix this by using `exit(1)` instead.

The "%p" formatter has platform-specific behaviour. On Windows for
example it prints as a zero-padded integer, whereas on glibc systems it
prints as non-zero-padded integer with "0x" prepended.

Address this by instead using PRIxPTR and casting the pointers to the
`uintptr_t` type.

The examples are closely coupled with our selftests, which makes it hard
to iterate on both. Split them out into their own standalone directory
as a prerequisite for wiring up proper testing of the clar.

We are about to wire up proper testing of clar via CI. This will be done
by executing the "clar_test" binary we already have in another clar test
binary. This requires us to move the current "clar_test" into its own
subdirectory such that "generate.py" can then generate the "clar.suite"
for each of these two test binaries independently.

The global test counter has been added as a demonstration for how it is
possible to modify global resources. Now that we have moved examples
into their own directory though it no longer serves that purpose.

In fact, it now stands in our way as we are about to introduce testing
of the clar itself with various different options. But as we assert that
the counter is "8" after all tests have ran, we wouldn't be able to
exclude some tests from running.

Drop the counter altogether to prepare for the change.

The output for the test that exercise `cl_assert_equal_p()` is
indeterministic because we compare on-stack addresses, which naturally
change between executions. Adapt the test to instead use fixed values.

While we have had the "clar_test" binary for a very long time already,
it was more for demonstration purposes than for real testing of the
clar. It could serve as sort of a smoke test if the developer remembered
to execute it, but that's about it.

In the preceding commits we have moved the "clar_test" into a separate
directory, renamed it to "selftest_suite" and made its output
deterministic. We can thus now wire up proper testing of the clar by
exercising the "selftest_suite" binary via a new "selftest" binary.
These tests run the "selftest_test" binary with various different
arguments and check both its return code as well as its output.

Wire up support for testing in our CI now that we have proper self
tests.