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.

toolchain/parser/parse_tree_file_test.cpp

@@ -16,11 +16,8 @@ class ParseTreeFileTest : public DriverFileTestBase {
  public:
   using DriverFileTestBase::DriverFileTestBase;
 
-  auto MakeArgs(const llvm::SmallVector<llvm::StringRef>& test_files)
-      -> llvm::SmallVector<llvm::StringRef> override {
-    llvm::SmallVector<llvm::StringRef> args({"dump", "parse-tree"});
-    args.insert(args.end(), test_files.begin(), test_files.end());
-    return args;
+  auto GetDefaultArgs() -> llvm::SmallVector<std::string> override {
+    return {"dump", "parse-tree", "%s"};
   }
 };