[FileCheck] Use lit's internal shell for the test suite

An advantage is that there are less portability concerns when writing
tests.  For example, `-u` is not supported by all implementations of
`env`, but lit's internal shell provides its own `env` that supports
`-u`.

A disadvantage is that some shell constructs, such as parentheses, are
not supported, but FileCheck's test suite currently doesn't require
such constructs.

For comparison, lit configures its test suite in the same manner.  See
`llvm/utils/lit/tests/lit.cfg`.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D70278