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?

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