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/codegen/testdata/objcode/basic.carbon

@@ -0,0 +1,10 @@
+// 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: dump objcode --target_triple=x86_64-unknown-linux-gnu --output_file=%t %s
+// TODO: Support autoupdate with ARGS.
+// NOAUTOUPDATE
+// CHECK:STDOUT: Success: Object file is generated!
+
+fn Main() -> i32 { return 0; }