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...