Fix unintended compile error in build.zig example

[generalmimon]

As you can see at https://web.archive.org/web/20250924165935/https://ziglang.org/learn/build-system/#testing, this example failed to compile with:

$ zig build test --summary all
/home/ci/actions-runner-website/_work/www.ziglang.org/www.ziglang.org/zig-code/build-system/unit-testing/build.zig:20:14: error: no field named 'root_source_file' in struct 'Build.TestOptions'
            .root_source_file = b.path("main.zig"),
             ^~~~~~~~~~~~~~~~
/home/ci/deps/zig-x86_64-linux-0.15.1/lib/std/Build.zig:856:25: note: struct declared here
pub const TestOptions = struct {
                        ^~~~~~

Despite the fact that this build.zig example apparently should fail to compile (since it contains build=fail at the end), this is clearly not the compile error that was intended. I'm sure of that because this example is followed by the diff that should be enough to fix the compile error:

www.ziglang.org/content/en-US/learn/build-system.smd

Lines 240 to 252 in a69386e

	In this case it might be a nice adjustment to enable `skip_foreign_checks` for
	the unit tests:
	```diff
	@@ -23,6 +23,7 @@
	});
	const run_unit_tests = b.addRunArtifact(unit_tests);
	+ run_unit_tests.skip_foreign_checks = true;
	test_step.dependOn(&run_unit_tests.step);
	}
	}
	```

Therefore, this commit fixes the above compile error by following the build.zig example with this skip_foreign_checks patch applied:

www.ziglang.org/zig-code/build-system/unit-testing-skip-foreign/build.zig

Lines 20 to 23 in a69386e

	.root_module = b.createModule(.{
	.root_source_file = b.path("main.zig"),
	.target = b.resolveTargetQuery(target),
	}),