testing: --run now allows filtering of subtests

Also fix typo in error message in sub_test.go from upstream,
and move a few members from B to common where they belonged.

Note that testdata/testing.go seems to be pushing the edge of what
the emulated cortex-m3 target can handle; using regexp in that test
causes it to fail on that target with an out of memory error.

TODO: once tinygo supports runtime.Goexit, consider just using upstream's testing directory...

src/testing/sub_test.go

@@ -32,7 +32,7 @@ func TestRunCleanup(t *T) {
 		t.Errorf("unexpected inner cleanup count; got %d want 1", innerCleanup)
 	}
 	if outerCleanup != 1 {
-		t.Errorf("unexpected outer cleanup count; got %d want 0", outerCleanup)
+		t.Errorf("unexpected outer cleanup count; got %d want 1", outerCleanup) // wrong upstream!
 	}
 }