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

[jdenny]

Summary

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.

[rnk]

lgtm

llvm/test/FileCheck/lit.local.cfg

+import lit
+
+# Use lit's internal shell to help guarantee test portability.
+config.test_format = lit.formats.ShTest(execute_external=False)
+
 # Unset environment variables that the FileCheck tests
 # expect not to be set.
 file_check_expected_unset_vars = [
   'FILECHECK_DUMP_INPUT_ON_FAILURE',
   'FILECHECK_OPTS',
 ]
 
 for env_var in file_check_expected_unset_vars:
   if env_var in config.environment:
     lit_config.note('Removing {} from environment for FileCheck tests'.format(
       env_var)
     )
     config.environment.pop(env_var)