cmd/go: avoid passing testing.T to isCaseSensitive and goVersion
The previous implementation of isCaseSensitive called t.Fatalf in the
wrong place, causing tests after the first to proceed past an error
determining case-sensitivity. That could lead to confusing errors.
(Moreover, I would like to try to disentangle the script engine from
testing.T so that I can also use it to generate serving contents in
the replacement for vcs-test.golang.org.)
The implementation of goVersion called ts.fatalf, which is probably
fine but prevents the script environment from being computed outside
of a test, as we might want to do for debugging and other scripting.