Add unit test for `command.ConfigUsed` (#2446)

## Changes
I forgot to add a unit test in the original PR, so adding it here:
https://github.com/databricks/cli/pull/2440

## Why
Unit testing code is a good practice. I believe we all should unit test
code.

## Tests
N/A

NO_CHANGELOG=true

libs/command/config_used_test.go

@@ -0,0 +1,37 @@
+package command
+
+import (
+	"context"
+	"testing"
+
+	"github.com/databricks/databricks-sdk-go/config"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCommandConfigUsed(t *testing.T) {
+	cfg := &config.Config{
+		Host: "https://test.com",
+	}
+	ctx := context.Background()
+
+	// Panic if ConfigUsed is called before SetConfigUsed.
+	assert.Panics(t, func() {
+		ConfigUsed(ctx)
+	})
+
+	ctx = SetConfigUsed(ctx, cfg)
+
+	// Multiple calls should return a pointer to the same config.
+	c := ConfigUsed(ctx)
+	assert.Same(t, c, ConfigUsed(ctx))
+
+	// The config should have the correct configuration.
+	assert.Equal(t, "https://test.com", ConfigUsed(ctx).Host)
+
+	// Second call should update the config used.
+	cfg2 := &config.Config{
+		Host: "https://test2.com",
+	}
+	ctx = SetConfigUsed(ctx, cfg2)
+	assert.Equal(t, "https://test2.com", ConfigUsed(ctx).Host)
+}