Add ability use script as cwd.

build.zig

@@ -63,4 +63,25 @@ pub fn build(b: *std.Build) void {
     run_cmd.step.dependOn(b.getInstallStep());
     if (b.args) |args| run_cmd.addArgs(args);
     b.step("run", "Run the app").dependOn(&run_cmd.step);
+
+    // Tests are rooted at Layouts.zig rather than main.zig: the parts of this
+    // app worth testing in isolation are the ones that are pure data — layout
+    // parsing, parameter substitution, the `$(...)` expansion and the script
+    // runner underneath it. Everything else is a widget tree, which wants a
+    // display and a person looking at it.
+    //
+    // Rooting there also keeps the test binary off GTK entirely; Layouts.zig
+    // and script.zig reach for GLib and nothing above it.
+    const tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/Layouts.zig"),
+            .target = target,
+            .optimize = optimize,
+            .link_libc = true,
+        }),
+    });
+    tests.root_module.addImport("glib", gobject.module("glib2"));
+
+    const run_tests = b.addRunArtifact(tests);
+    b.step("test", "Run the tests").dependOn(&run_tests.step);
 }