Fix LIT test breakage

[george.karpenkov]

Summary

Addresses the issue in https://reviews.llvm.org/D35091

At least with the current change it's trivial to test:

LIT_FILTER=selecting.py ./lit.py -v tests

[modocache]

Great! This gets the test passing for me when I run on my macOS development machine. Thanks!

[mgorny]

Green on Linux as well. I suppose Windows will be broken but we can mark it appropriately when the buildbots are up and confirm.

llvm/trunk/utils/lit/tests/selecting.py

 # RUN: %{lit} %{inputs}/discovery | FileCheck --check-prefix=CHECK-BASIC %s
 # CHECK-BASIC: Testing: 5 tests
 
 # PR33937
 # XFAIL: windows
 
 # Check that regex-filtering works
 #
 # RUN: %{lit} --filter 'o[a-z]e' %{inputs}/discovery | FileCheck --check-prefix=CHECK-FILTER %s
 # CHECK-FILTER: Testing: 2 of 5 tests
 
 # Check that regex-filtering based on environment variables work.
 #
-# RUN: LIT_FILTER='o[a-z]e' %{lit} %{inputs}/discovery | FileCheck --check-prefix=CHECK-FILTER-ENV %s
+# RUN: env LIT_FILTER='o[a-z]e' %{lit} %{inputs}/discovery | FileCheck --check-prefix=CHECK-FILTER-ENV %s
 # CHECK-FILTER-ENV: Testing: 2 of 5 tests
 
 
 # Check that maximum counts work
 #
 # RUN: %{lit} --max-tests 3 %{inputs}/discovery | FileCheck --check-prefix=CHECK-MAX %s
 # CHECK-MAX: Testing: 3 of 5 tests