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_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/global_exe_path.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 = GetExePath();
+  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