Typechecker tests failures

[Yuras Shumovich]

Hello,

I was working on #9605, and found a number of failed tests:

Unexpected failures:
   should_compile  T7891 [exit code non-0] (hpc,optasm,optllvm)
   should_compile  tc124 [exit code non-0] (hpc,optasm,optllvm)
   should_run      T7861 [bad exit code]
(normal,hpc,optasm,ghci,threaded1,threaded2,dyn,optllvm)

They seem to be unrelated to my work, and they are skipped when "fast"
is enabled.

T7891 and tc124 fail with Core Lint errors.

Looks like Phabricator uses "fast" way to validate revisions, so the
failures were not noticed.

Thanks,
Yuras