Add support for including another test file at a point in the current
test file with ** include: file.t

The included file is parsed recursively — its tests, directives, and
nested includes take effect as if pasted inline at the ** include:
line. Paths are resolved relative to the including file's directory.

Multiple ** include: lines are needed to include several files; each
line includes exactly one file.

- Circular include chains are detected and raise an error with the
  full cycle path (e.g. A -> B -> C -> A)
- Duplicate test names across files raise an error showing both
  source files
- All directives follow the same rules: ** game: overrides,
  ** precommand: appends, etc.

>{include:silent} TESTNAME runs the included test's commands as usual but
keeps their output out of the verbose transcript, in order to avoid repetition
in the test transcripts.
The included test's checks still run, and their failures are still reported.
Pass -vv to see everything anyway.

In -v mode a marker line, [silently included: TESTNAME], is printed in
place of the hidden block, so the transcript still shows where the
included test ran.

Signed-off-by: Samuel Verschelde <stormi-git@ylix.fr>

When the first thing a test does is silenced, whatever runs before it is
setup you have already seen too: the game's initial output, and any
precommands ahead of it. Those are now hidden as well, so the transcript
of each test begins where the test itself does.

A test starts silently when its first command is >{include:silent}, when
it plainly includes a wrapper test which itself starts silently, or when
the silent include is a precommand, in which case it is the first thing
every test in the file does.

poll() is a non-blocking check that often runs before the process
has actually finished dying, so the exit status is never collected
and the child is left as a zombie.

wait() ensures the child process is collected.

Signed-off-by: Samuel Verschelde <stormi-git@ylix.fr>