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.cpp

@@ -2,11 +2,34 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include "testing/base/gtest_main.h"
+
 #include <gtest/gtest.h>
 
+#include <string>
+
+#include "common/check.h"
+#include "common/exe_path.h"
 #include "common/init_llvm.h"
 
+static bool after_main = false;
+static llvm::StringRef exe_path;
+
+namespace Carbon::Testing {
+
+auto GetTestExePath() -> llvm::StringRef {
+  CARBON_CHECK(after_main)
+      << "Must not query the executable path until after `main` is entered!";
+  return exe_path;
+}
+
+}  // namespace Carbon::Testing
+
 auto main(int argc, char** argv) -> int {
+  std::string exe_path_storage = Carbon::FindExecutablePath(argv[0]);
+  exe_path = exe_path_storage;
+  after_main = true;
+
   Carbon::InitLLVM init_llvm(argc, argv);
   testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();