tests: ASSERT_SIGNAL: Prevent hallucinating parent as child and confusing exit codes...

This series fixes two distinct, pretty bad bugs in `ASSERT_SIGNAL`.
These bugs can allow failing tests to pass, and can also cause the test
runner to silently terminate prematurely in a way that looks like
success.

This is not theoretical, see
https://github.com/systemd/systemd/pull/39674#discussion_r2540552699 for
a real case of this happening.

---

Bug 1: Parent process hallucinates it is the child and re-executes the
expression being tested

Previously, assert_signal_internal() returned 0 in two mutually
exclusive states:

1. We are the child process (immediately after fork()).
2. We are the parent process, and the child exited normally (status 0).

The macro failed to distinguish these cases. If a child failed to crash
as expected, the parent received 0, incorrectly interpreted it as it
being the child, and re-executed the test expression inside the parent
process.

This can cause tests to falsely pass. The parent would successfully run
the expression (which wasn't supposed to crash in the parent), succeed,
and call _exit(EXIT_SUCCESS).

The second consequence is silent truncation. When the parent called
_exit(), it terminated the entire test runner immediately. Any
subsequent tests in the same binary were never executed.

---

Bug 2: Conflation of exit codes and signals

The harness returned the raw si_status without checking si_code. This
meant that an exit code was indistinguishable from a signal number. For
example, if a child process failed and called exit(6), the harness
reported it as having been killed by SIGABRT (signal 6).

---

This PR both fixes the bugs and reworks the ASSERT_SIGNAL infrastructure
to ensure this is very unlikely to regress:

- assert_signal_internal now returns an explicit control flow enum
(FORK_CHILD / FORK_PARENT) separate from the status data. This makes it
structurally impossible for the parent to hallucinate that it is the
child.
- The output parameter is only populated with a signal number if si_code
confirms the process was killed by a signal. Normal exits return 0.