libstore-tests: Split bench-main into a separate file

This makes it easier to add new benchmarks.

src/libstore-tests/bench-main.cc

@@ -0,0 +1,14 @@
+#include <benchmark/benchmark.h>
+#include "nix/store/globals.hh"
+
+// Custom main to initialize Nix before running benchmarks
+int main(int argc, char ** argv)
+{
+    // Initialize libstore
+    nix::initLibStore(false);
+
+    // Initialize and run benchmarks
+    ::benchmark::Initialize(&argc, argv);
+    ::benchmark::RunSpecifiedBenchmarks();
+    return 0;
+}