Add file_test support for specifying arguments and only checking a subset of output. (#3018)

This is sufficient to convert remaining toolchain tests to file_test. %s
and %t are currently used. While #2978 might use %T with lit, I think
that's resolving a lit-specific issue that's not necessary when
TEST_TMPDIR is readily available for any necessary operations (i.e.,
setting the working directory). With ARGS, I think it's feasible to
switch to file_test without %T.

Also adds documentation to file_test_base.h, which was starting to feel
like a significant gap.

This is not yet handled by autoupdate, but I'm eyeing that next.

testing/file_test/testdata/fail_example.carbon

@@ -2,4 +2,5 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+// CHECK:STDOUT: 2 args: `default_args`, `fail_example.carbon`
 // CHECK:STDERR: Oops