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/global_exe_path.cpp

@@ -0,0 +1,27 @@
+// 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/global_exe_path.h"
+
+#include <string>
+
+#include "common/check.h"
+#include "common/exe_path.h"
+
+static constinit std::optional<std::string> exe_path = {};
+
+namespace Carbon::Testing {
+
+auto GetExePath() -> llvm::StringRef {
+  CARBON_CHECK(exe_path)
+      << "Must not query the executable path until after it has been set!";
+  return *exe_path;
+}
+
+auto SetExePath(const char* argv_zero) -> void {
+  CARBON_CHECK(!exe_path) << "Must not call `SetExePath` more than once!";
+  exe_path.emplace(Carbon::FindExecutablePath(argv_zero));
+}
+
+}  // namespace Carbon::Testing