Use `--allow-unused-prefixes=false` by default for FileCheck in MLIR testsuite

This option catches unexpected mismatch when a prefix is given to
FileCheck on the command line but never matches a single line in the
test.

See http://lists.llvm.org/pipermail/llvm-dev/2020-October/146162.html
for more info.

Differential Revision: https://reviews.llvm.org/D90501