[libc] Support running tests for Pigweed

[michaelrj]

LGTM

libc/test/UnitTest/PigweedTest.h

-//===-- Header selector for libc unittests ----------------------*- C++ -*-===//
+//===-- Header for setting up the Pigweed tests -----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_LIBC_UTILS_UNITTEST_TEST_H
-#define LLVM_LIBC_UTILS_UNITTEST_TEST_H
+#ifndef LLVM_LIBC_UTILS_UNITTEST_PIGWEEDTEST_H
+#define LLVM_LIBC_UTILS_UNITTEST_PIGWEEDTEST_H
 
-#ifdef LIBC_COPT_TEST_USE_FUCHSIA
-#include "FuchsiaTest.h"
-#else
-#include "LibcTest.h"
-#endif
+#include <gtest/gtest.h>
 
-#endif // LLVM_LIBC_UTILS_UNITTEST_TEST_H
+namespace __llvm_libc::testing {
+using Test = ::testing::Test;
+}
+
+#endif // LLVM_LIBC_UTILS_UNITTEST_PIGWEEDTEST_H

libc/test/UnitTest/Test.h

 //===-- Header selector for libc unittests ----------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_UTILS_UNITTEST_TEST_H
 #define LLVM_LIBC_UTILS_UNITTEST_TEST_H
 
 #ifdef LIBC_COPT_TEST_USE_FUCHSIA
 #include "FuchsiaTest.h"
+#elif defined(LIBC_COPT_TEST_USE_PIGWEED)
+#include "PigweedTest.h"
 #else
 #include "LibcTest.h"
 #endif
 
 #endif // LLVM_LIBC_UTILS_UNITTEST_TEST_H