reset plan changes every call to Plan.

Tests often call Plan multiple times on the same context to verify there
are no changes, so we need to make sure changes don't accumulate.

terraform/context.go

@@ -489,6 +489,7 @@ func (c *Context) Apply() (*states.State, tfdiags.Diagnostics) {
 // by the plan, so Apply can be called after.
 func (c *Context) Plan() (*plans.Plan, tfdiags.Diagnostics) {
 	defer c.acquireRun("plan")()
+	c.changes = plans.NewChanges()
 
 	var diags tfdiags.Diagnostics