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.

explorer/file_test.cpp

@@ -42,9 +42,13 @@ class ParseAndExecuteTestFile : public FileTestBase {
     }
   }
 
-  auto RunWithFiles(const llvm::SmallVector<TestFile>& test_files,
+  auto RunWithFiles(const llvm::SmallVector<llvm::StringRef>& test_args,
+                    const llvm::SmallVector<TestFile>& test_files,
                     llvm::raw_pwrite_stream& stdout,
                     llvm::raw_pwrite_stream& stderr) -> bool override {
+    CARBON_CHECK(test_args.empty())
+        << "ARGS are not currently used in explorer's file_test.";
+
     if (test_files.size() != 1) {
       ADD_FAILURE() << "Only 1 file is supported: " << test_files.size()
                     << " provided";
@@ -88,6 +92,10 @@ class ParseAndExecuteTestFile : public FileTestBase {
     return result.ok();
   }
 
+  auto GetDefaultArgs() -> llvm::SmallVector<std::string> override {
+    return {};
+  }
+
  private:
   bool trace_;
   bool is_trace_test = false;