Add a main function to tests to ensure the package doesn't include any other entrypoints

Author: bourdakos1

Package.swift

@@ -22,7 +22,7 @@ let package = Package(
         // main functions
         "absl/hash/internal/print_hash_of.cc",
         "absl/random/internal/gaussian_distribution_gentables.cc",
-        "absl/random/internal/randen_benchmarks.cc",
+        // "absl/random/internal/randen_benchmarks.cc",
       ],
       sources: [
         "absl/"

SwiftPMTests/build-test/test.cc

@@ -46,3 +46,6 @@
 #include "absl/types/any.h"
 #include "absl/types/optional.h"
 #include "absl/types/variant.h"
+
+// Test for duplicate `_main` symbol.
+int main(int argc, char** argv) {}