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.

common/exe_path_test.cpp

@@ -0,0 +1,47 @@
+// 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 "common/exe_path.h"
+
+#include <gtest/gtest.h>
+
+#include "llvm/ADT/SmallString.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
+
+namespace Carbon {
+namespace {
+
+TEST(ExePath, FailureFallback) {
+  llvm::SmallString<128> path = llvm::StringRef(getenv("TEST_TMPDIR"));
+  llvm::sys::path::append(path, "non_existant_binary");
+  std::string exe_path = FindExecutablePath(path);
+  EXPECT_EQ(path, exe_path);
+}
+
+TEST(ExePath, File) {
+  llvm::SmallString<128> path = llvm::StringRef(getenv("TEST_TMPDIR"));
+  llvm::sys::path::append(path, "test_binary");
+  int fd = -1;
+  std::error_code ec = llvm::sys::fs::openFileForWrite(path, fd);
+  ASSERT_TRUE(!ec) << "Error code: " << ec;
+  close(fd);
+
+  std::string exe_path = FindExecutablePath(path);
+  EXPECT_EQ(path, exe_path);
+}
+
+TEST(ExePath, PathLookup) {
+  // TODO: This is not likely to work well on Windows (outside of WSL). But some
+  // of that may be hidden by Bazel's test environment. Regardless, we should
+  // revisit this when we have good coverage of Windows build with something
+  // appropriate for that platform.
+  std::string exe_path = FindExecutablePath("bash");
+  EXPECT_NE(exe_path, "bash");
+  EXPECT_TRUE(llvm::sys::fs::exists(exe_path));
+  EXPECT_TRUE(llvm::sys::fs::can_execute(exe_path));
+}
+
+}  // namespace
+}  // namespace Carbon