add the new extern test to standalone tests

This was from master branch commit
c93e0d8618. Since standalone test are
completely reworked, I had to resolve the merge conflict later, in this
commit.

test/standalone.zig

@@ -147,6 +147,10 @@ pub const build_cases = [_]BuildCase{
         .build_root = "test/standalone/embed_generated_file",
         .import = @import("standalone/embed_generated_file/build.zig"),
     },
+    .{
+        .build_root = "test/standalone/extern",
+        .import = @import("standalone/extern/build.zig"),
+    },
     .{
         .build_root = "test/standalone/dep_diamond",
         .import = @import("standalone/dep_diamond/build.zig"),

test/standalone/extern/build.zig

@@ -1,7 +1,7 @@
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
-    const optimize = b.standardOptimizeOption(.{});
+    const optimize: std.builtin.OptimizeMode = .Debug;
 
     const obj = b.addObject(.{
         .name = "exports",