test: add the pwtest test framework

Heavily inspired by libinput's litest framework (built around check), this is
a from-scratch framework that simplifies adding tests for various parts of
pipewire. See the pwtest.h documentation for details but the basics are:

- PW_TEST() and PWTEST_SUITE() specify the tests to be run
- Test are run in forked processes, any errors/signals are caught and printed
  to the log
- Tests have a custom pipewire daemon started on demand to talk to [1]. The
  daemon's log is available in the test output.
- Output is YAML to be processed into whatever format needed

[1] There are limits here, since we can't emulate devices yet there is only
so much we can rely on with the daemon.

spa/include/spa/utils/defs.h

@@ -200,6 +200,7 @@ struct spa_fraction {
 #define SPA_EXPORT __attribute__((visibility("default")))
 #define SPA_SENTINEL __attribute__((__sentinel__))
 #define SPA_UNUSED __attribute__ ((unused))
+#define SPA_NORETURN __attribute__ ((noreturn))
 #else
 #define SPA_PRINTF_FUNC(fmt, arg1)
 #define SPA_ALIGNED(align)
@@ -207,6 +208,7 @@ struct spa_fraction {
 #define SPA_EXPORT
 #define SPA_SENTINEL
 #define SPA_UNUSED
+#define SPA_NORETURN
 #endif
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L