build-system: Replace deprecated usages of root_source_file on addTests

pkg/cimgui/build.zig

@@ -108,9 +108,11 @@ pub fn build(b: *std.Build) !void {
 
     const test_exe = b.addTest(.{
         .name = "test",
-        .root_source_file = b.path("main.zig"),
-        .target = target,
-        .optimize = optimize,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("main.zig"),
+            .target = target,
+            .optimize = optimize,
+        }),
     });
     test_exe.linkLibrary(lib);
     const tests_run = b.addRunArtifact(test_exe);