Allow GoogleTest tests to locate their executable path. (#3992)

This also factors out the code for doing this location from the driver
to a tiny helper library.

The motivation for this is letting tests locate data files like the
prelude or other files needed by the toolchain more easily. A subsequent
PR will use this heavily in the Clang runner and related logic for
example.

testing/base/gtest_main_test.cpp

@@ -0,0 +1,25 @@
+// 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 "testing/base/gtest_main.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "llvm/Support/FileSystem.h"
+
+namespace Carbon::Testing {
+namespace {
+
+using ::testing::StrNe;
+
+TEST(TestExePathTest, Test) {
+  llvm::StringRef exe_path = GetTestExePath();
+  EXPECT_THAT(exe_path, StrNe(""));
+  EXPECT_TRUE(llvm::sys::fs::exists(exe_path));
+  EXPECT_TRUE(llvm::sys::fs::can_execute(exe_path));
+}
+
+}  // namespace
+}  // namespace Carbon::Testing