Makefile: bump the test timeout

The test suite has progressively become slower and we're now running
into the 8m limits, causing lots of test flakes.

This new default will bring us forward, but really a more durable
solution would be to profile the tests and divide the work among more
CI tasks running in parallel.

Release note: None

Makefile

@@ -63,7 +63,7 @@ TESTCONFIG :=
 SUBTESTS :=
 
 ## Test timeout to use for regular tests.
-TESTTIMEOUT := 8m
+TESTTIMEOUT := 12m
 
 ## Test timeout to use for race tests.
 RACETIMEOUT := 25m