meson: Disable all tests when tests are disabled

meson: Disable all tests when tests are disabled

Currently, passing -Dtests=disabled only disables a subset of
tests: those that are written in C and thus require compilation.
Other tests, such as the syntax-check ones and those that are
implemented as scripts, are always enabled.

There's a potentially dangerous consequence of this behavior:
when tests are disabled, 'meson test' will succeed as if they
had been enabled. No indication of this will be shown, so the
user will likely make the reasonable assumption that everything
is fine when in fact the significantly reduced coverage might
be hiding failures.

To solve this issues, disable *all* tests when asked to do so,
and inject an intentionally failing test to ensure that 'meson
test' doesn't succeed.

Best viewed with 'git show -w'.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>