LPS-51151 Add unit tests for SchedulerClusterInvokeAcceptor.accept

Hi Tina, your last commit 950cc10 caused ClusterSchedulerEngineTest.testSetClusterableThreadLocal2 to fail.

```
junit.framework.AssertionFailedError
	at com.liferay.portal.scheduler.ClusterSchedulerEngineTest.testSetClusterableThreadLocal2(ClusterSchedulerEngineTest.java:628)
```
The logic changed here:
**Old code**:
```java
if (ClusterInvokeThreadLocal.isEnabled()) {
	return true;
}
```
**new code**:
```java
if (!ClusterInvokeThreadLocal.isEnabled()) {
	return false;
}
```
The test is verifying this:
```java
Assert.assertTrue(_clusterInvokeAcceptor.accept(context));
```
Could you take a look on it? 'cause maybe the test assertion is no longer valid.