Add coverage testing for parse node kinds. (#4436)

This refactors the diagnostic kind coverage check into something that
also works for node kinds. Then, since this points out a few node kinds
that aren't having their parse verified, I'm adding minor tests for
those.

toolchain/parse/coverage_test.cpp

@@ -0,0 +1,32 @@
+// 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
+
+#include <gtest/gtest.h>
+
+#include "absl/flags/flag.h"
+#include "toolchain/parse/node_kind.h"
+#include "toolchain/testing/coverage_helper.h"
+
+ABSL_FLAG(std::string, testdata_manifest, "",
+          "A path to a file containing repo-relative names of test files.");
+
+namespace Carbon::Parse {
+namespace {
+
+constexpr NodeKind NodeKinds[] = {
+#define CARBON_PARSE_NODE_KIND(Name) NodeKind::Name,
+#include "toolchain/parse/node_kind.def"
+};
+
+constexpr NodeKind UntestedNodeKinds[] = {NodeKind::Placeholder};
+
+// Looks for node kinds that aren't covered by a file_test.
+TEST(Coverage, NodeKind) {
+  Testing::TestKindCoverage(absl::GetFlag(FLAGS_testdata_manifest),
+                            R"(kind: '(\w+)')", llvm::ArrayRef(NodeKinds),
+                            llvm::ArrayRef(UntestedNodeKinds));
+}
+
+}  // namespace
+}  // namespace Carbon::Parse