Refactor testing exe path and benchmark main handling. (#4216)

Consolidates both main libraries into `//testing/base`, and factors out
the exe path handling for benchmarks and unit tests into a common
library to remove duplication. Refactors how that logic is managed to be
cleaner and avoid a confusing bool that came up in code review.

Updates all the tests and benchmarks that use these. I still need to
update other benchmarks to use the same main, but I wanted to keep this
PR somewhat minimal.

This also fixes a bug noticed in passing that the compilation benchmark
didn't have the required dependency on the benchmark library itself,
just the benchmark main library.

testing/base/gtest_main.cpp

@@ -2,35 +2,16 @@
 // 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
+#include "testing/base/global_exe_path.h"
 
 auto main(int argc, char** argv) -> int {
-  std::string exe_path_storage = Carbon::FindExecutablePath(argv[0]);
-  exe_path = exe_path_storage;
-  after_main = true;
-
+  // Initialize LLVM first, as that will also handle ensuring UTF-8 encoding.
   Carbon::InitLLVM init_llvm(argc, argv);
+
+  Carbon::Testing::SetExePath(argv[0]);
   testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();
 }