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/testing/file_test.cpp

@@ -23,9 +23,9 @@ namespace {
 // phase subdirectories.
 class ToolchainFileTest : public FileTestBase {
  public:
-  explicit ToolchainFileTest(llvm::StringRef exe_path,
+  explicit ToolchainFileTest(llvm::StringRef exe_path, std::mutex* output_mutex,
                              llvm::StringRef test_name)
-      : FileTestBase(test_name),
+      : FileTestBase(output_mutex, test_name),
         component_(GetComponent(test_name)),
         installation_(InstallPaths::MakeForBazelRunfiles(exe_path)) {}