GNU make is perfectly happy to use 'check-FOO-y += bar' to
initialize check-FOO-y.  (GNU Automake strictly insists that
you cannot use += until after an initial = per variable, but
thankfully we aren't using automake).

As we have had more than one instance where copy-and-paste of
'check-FOO-y = bar' from a first test under category FOO into
an additional test, which ends up disabling the first (see
commits 992159c7 and 4429532b), it's better to just always use
the form that survives copy-and-paste, even for categories that
don't currently add more than one test.

Done with s/^\(check-[a-z]*-y \)=/\1+=/g
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>