Fix flaky tests for the parallel mutator (#1426)

## Changes
Around 0.5% to 1% of the time, the tests would fail due to concurrent
access to the underlying slice in the mutator. This PR makes the test
thread safe preventing race conditions.

Example of failed run:
https://github.com/databricks/cli/actions/runs/9004657555/job/24738145829

bundle/parallel_test.go

@@ -2,6 +2,7 @@ package bundle
 
 import (
 	"context"
+	"sync"
 	"testing"
 
 	"github.com/databricks/cli/bundle/config"
@@ -10,9 +11,14 @@ import (
 )
 
 type addToContainer struct {
+	t         *testing.T
 	container *[]int
 	value     int
 	err       bool
+
+	// mu is a mutex that protects container. It is used to ensure that the
+	// container slice is only modified by one goroutine at a time.
+	mu *sync.Mutex
 }
 
 func (m *addToContainer) Apply(ctx context.Context, b ReadOnlyBundle) diag.Diagnostics {
@@ -20,9 +26,10 @@ func (m *addToContainer) Apply(ctx context.Context, b ReadOnlyBundle) diag.Diagn
 		return diag.Errorf("error")
 	}
 
-	c := *m.container
-	c = append(c, m.value)
-	*m.container = c
+	m.mu.Lock()
+	*m.container = append(*m.container, m.value)
+	m.mu.Unlock()
+
 	return nil
 }
 
@@ -36,9 +43,10 @@ func TestParallelMutatorWork(t *testing.T) {
 	}
 
 	container := []int{}
-	m1 := &addToContainer{container: &container, value: 1}
-	m2 := &addToContainer{container: &container, value: 2}
-	m3 := &addToContainer{container: &container, value: 3}
+	var mu sync.Mutex
+	m1 := &addToContainer{t: t, container: &container, value: 1, mu: &mu}
+	m2 := &addToContainer{t: t, container: &container, value: 2, mu: &mu}
+	m3 := &addToContainer{t: t, container: &container, value: 3, mu: &mu}
 
 	m := Parallel(m1, m2, m3)
 
@@ -57,9 +65,10 @@ func TestParallelMutatorWorkWithErrors(t *testing.T) {
 	}
 
 	container := []int{}
-	m1 := &addToContainer{container: &container, value: 1}
-	m2 := &addToContainer{container: &container, err: true, value: 2}
-	m3 := &addToContainer{container: &container, value: 3}
+	var mu sync.Mutex
+	m1 := &addToContainer{container: &container, value: 1, mu: &mu}
+	m2 := &addToContainer{container: &container, err: true, value: 2, mu: &mu}
+	m3 := &addToContainer{container: &container, value: 3, mu: &mu}
 
 	m := Parallel(m1, m2, m3)