vecmath: Update to zig 0.16.0, harden tests and add test build step

packages/vecmath/build.zig

@@ -1,7 +1,19 @@
 const std = @import("std");
 
 pub fn build(b: *std.Build) void {
-    _ = b.addModule("vecmath", .{
+    const target = b.standardTargetOptions(.{});
+
+    const mod = b.addModule("vecmath", .{
         .root_source_file = b.path("src/root.zig"),
+        .target = target,
     });
+
+    const mod_tests = b.addTest(.{
+        .root_module = mod,
+    });
+
+    const run_mod_tests = b.addRunArtifact(mod_tests);
+
+    const test_step = b.step("test", "Run tests");
+    test_step.dependOn(&run_mod_tests.step);
 }