add syntax sugar for cleanup tasks related to test failures

Conditional cleanups are particularly helpful
for logging specific details only when a test fails,
preventing unnecessary clutter in the output of passing tests.

T.go

@@ -296,3 +296,11 @@ func (t *T) LogPretty(vs ...any) {
 func (t *T) Done() <-chan struct{} {
 	return t.done
 }
+
+func (t *T) OnFail(fn func()) {
+	t.Defer(func() {
+		if t.Failed() {
+			fn()
+		}
+	})
+}

T_test.go

@@ -808,3 +808,43 @@ func TestT_Done(t *testing.T) {
 		assert.Equal(t, atomic.LoadInt32(&done), 1)
 	})
 }
+
+func TestT_OnFail(t *testing.T) {
+	t.Run("on success", func(t *testing.T) {
+		dtb := &doubles.TB{}
+		var done bool
+		s := testcase.NewSpec(dtb)
+		s.Test("", func(t *testcase.T) {
+			t.OnFail(func() { done = true })
+		})
+		s.Finish()
+		dtb.Finish()
+		assert.Equal(t, false, done)
+	})
+	t.Run("on failure", func(t *testing.T) {
+		dtb := &doubles.TB{}
+		var done bool
+		s := testcase.NewSpec(dtb)
+		s.Test("", func(t *testcase.T) {
+			t.OnFail(func() { done = true })
+			t.FailNow()
+		})
+		s.Finish()
+		dtb.Finish()
+		assert.Equal(t, true, done)
+	})
+	t.Run("race", func(t *testing.T) {
+		dtb := &doubles.TB{}
+		s := testcase.NewSpec(dtb)
+		s.Test("", func(t *testcase.T) {
+			testcase.Race(func() {
+				t.OnFail(func() {})
+			}, func() {
+				t.OnFail(func() {})
+			})
+			t.FailNow()
+		})
+		s.Finish()
+		dtb.Finish()
+	})
+}

examples_test.go

@@ -1576,3 +1576,17 @@ func ExampleSpec_Spec() {
 		sharedSuite.AsSuite("x").Benchmark(b)
 	}
 }
+
+func ExampleT_OnFail() {
+	s := testcase.NewSpec(nil)
+
+	s.Before(func(t *testcase.T) {
+		t.OnFail(func() {
+			// executes only when a test fails.
+		})
+	})
+
+	s.Test("", func(t *testcase.T) {
+		t.FailNow()
+	})
+}