Add support for capturing console output to FileTest. (#4339)

One of the things that ClangRunnerTest is doing is capturing
stderr/stdout because clang prints to it directly. This adds support for
that to FileTest.

I'm renaming the current `capture_output` field to `dump_output` because
the name is ambiguous after this change, and the flag is already named
`--dump_output`. It's still not great, but at least it's more distinct.

Note ClangRunner still doesn't use the vfs; that still needs work. I'm
just moving the NoArgs test over as a trivial test of the functionality.

toolchain/driver/testdata/fail_clang_no_args.cpp

@@ -0,0 +1,14 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// ARGS: clang --
+//
+// SET-CAPTURE-CONSOLE-OUTPUT
+// clang-format off
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/driver/testdata/fail_clang_no_args.cpp
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/driver/testdata/fail_clang_no_args.cpp
+// CHECK:STDERR: error: no input files