[lit] Check for accidental external command calls

This patch extends lit's test suite to check that lit's internal shell
doesn't accidentally execute internal commands as external commands.
It does so by putting fake failing versions of those commands in
`PATH` while the entire lit test suite is running.  Without the fixes
in D65697 but with its tests, this approach catches accidental
external `env` calls.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D66293

llvm-svn: 369309