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...

testdata/testing.txt

@@ -12,5 +12,10 @@
         failed
         after failed
     log Bar end
+--- FAIL: TestAllLowercase (0.00s)
+    --- FAIL: TestAllLowercase/BETA (0.00s)
+        expected lowercase name, got BETA
+    --- FAIL: TestAllLowercase/DELTA (0.00s)
+        expected lowercase name, got DELTA
 FAIL
 exitcode: 1