unittests: make ninja detection faster and more robust

Instead of blindly assuming when $CI is set that `ninja` is always
correct and always new enough, check for it the same way we do when $CI
is not set.

Instead of special casing when $CI is set and skipping ninja detection
in subprocess tests (by assuming it is always `ninja`), skip detection
in subprocess tests all the time, by passing this information around
across processes after the first time it is looked up.

This means that local (non-CI) tests are faster too!

Fixes running unittests in alpine linux using samu. Although ninja is a
symlink to samu, the exact output string on no-op builds is different,
which we already handle, but what we don't handle is the fact that samu
prints a third case when you invoke it as `ninja`. If $CI is exported,
then the unittests ignored $NINJA entirely.

Fixes running unittests when $CI is set, `samu` exists and `ninja` does
not exist. This is not currently the case anywhere, but may be in the
future; why not fix it now?

run_tests.py

@@ -46,13 +46,13 @@ from mesonbuild.mesonlib import OptionKey, setup_vsenv
 
 NINJA_1_9_OR_NEWER = False
 NINJA_CMD = None
-# If we're on CI, just assume we have ninja in PATH and it's new enough because
-# we provide that. This avoids having to detect ninja for every subprocess unit
-# test that we run.
-if 'CI' in os.environ:
-    NINJA_1_9_OR_NEWER = True
-    NINJA_CMD = ['ninja']
-else:
+# If we're on CI, detecting ninja for every subprocess unit test that we run is slow
+# Optimize this by respecting $NINJA and skipping detection, then exporting it on
+# first run.
+try:
+    NINJA_1_9_OR_NEWER = bool(int(os.environ['NINJA_1_9_OR_NEWER']))
+    NINJA_CMD = [os.environ['NINJA']]
+except (KeyError, ValueError):
     # Look for 1.9 to see if https://github.com/ninja-build/ninja/issues/1219
     # is fixed
     NINJA_CMD = detect_ninja('1.9')
@@ -61,7 +61,11 @@ else:
     else:
         mlog.warning('Found ninja <1.9, tests will run slower', once=True)
         NINJA_CMD = detect_ninja()
-if NINJA_CMD is None:
+
+if NINJA_CMD is not None:
+    os.environ['NINJA_1_9_OR_NEWER'] = str(int(NINJA_1_9_OR_NEWER))
+    os.environ['NINJA'] = NINJA_CMD[0]
+else:
     raise RuntimeError('Could not find Ninja v1.7 or newer')
 
 def guess_backend(backend_str: str, msbuild_exe: str) -> T.Tuple['Backend', T.List[str]]: