chore: add testutil.Eventually and friends (#3389)

This PR adds a `testutil` function aimed to replace `require.Eventually`.

Before:
```go
require.Eventually(t, func() bool { ... }, testutil.WaitShort, testutil.IntervalFast)
```

After:
```go
require.True(t, testutil.EventuallyShort(t, func(ctx context.Context) bool { ... }))

// or the full incantation if you need more control
ctx, cancel := context.WithTimeout(ctx.Background(), testutil.WaitLong)
require.True(t, testutil.Eventually(t, ctx, func(ctx context.Context) bool { ... }, testutil.IntervalSlow))
```

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

testutil/eventually_test.go

@@ -0,0 +1,70 @@
+package testutil_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.uber.org/goleak"
+
+	"github.com/coder/coder/testutil"
+)
+
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m)
+}
+
+func TestEventually(t *testing.T) {
+	t.Parallel()
+	t.Run("OK", func(t *testing.T) {
+		t.Parallel()
+		state := 0
+		condition := func(_ context.Context) bool {
+			defer func() {
+				state++
+			}()
+			return state > 2
+		}
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
+		defer cancel()
+		testutil.Eventually(ctx, t, condition, testutil.IntervalFast)
+	})
+
+	t.Run("Timeout", func(t *testing.T) {
+		t.Parallel()
+		condition := func(_ context.Context) bool {
+			return false
+		}
+		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
+		defer cancel()
+		mockT := new(testing.T)
+		testutil.Eventually(ctx, mockT, condition, testutil.IntervalFast)
+		assert.True(t, mockT.Failed())
+	})
+
+	t.Run("Panic", func(t *testing.T) {
+		t.Parallel()
+
+		panicky := func() {
+			mockT := new(testing.T)
+			condition := func(_ context.Context) bool { return true }
+			testutil.Eventually(context.Background(), mockT, condition, testutil.IntervalFast)
+		}
+		assert.Panics(t, panicky)
+	})
+
+	t.Run("Short", func(t *testing.T) {
+		t.Parallel()
+		testutil.EventuallyShort(t, func(_ context.Context) bool { return true })
+	})
+
+	t.Run("Medium", func(t *testing.T) {
+		t.Parallel()
+		testutil.EventuallyMedium(t, func(_ context.Context) bool { return true })
+	})
+
+	t.Run("Long", func(t *testing.T) {
+		t.Parallel()
+		testutil.EventuallyLong(t, func(_ context.Context) bool { return true })
+	})
+}