Migrate //common test libraries to //testing/util. (#2890)

This is just a cleanup. Since we now have a testing directory, I think this is a better home for testonly libraries than //common. (I was thinking about this when I was considering adding more test_raw_ostream deps)

testing/util/gtest_main.cpp

@@ -0,0 +1,17 @@
+// 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 "llvm/Support/InitLLVM.h"
+
+auto main(int argc, char** argv) -> int {
+  testing::InitGoogleTest(&argc, argv);
+  llvm::setBugReportMsg(
+      "Please report issues to "
+      "https://github.com/carbon-language/carbon-lang/issues and include the "
+      "crash backtrace.\n");
+  llvm::InitLLVM init_llvm(argc, argv);
+  return RUN_ALL_TESTS();
+}