[JENKINS-25514] Fixed lock case in FutureImpl

when some other process is waiting for build in queue.

https://issues.jenkins-ci.org/browse/JENKINS-25514
Test procedute:
1. Start some build using CLI with '-s' option and make sure that this build will stay in queue for some time (offline/busy executor etc)
2. Cancel the build from queue
=>
Expected result: the build is canceled and CLI dropped connection
Result before the fix: the build is canceled and CLI is still waiting without dropping connection.

You are right, I just removed unnecessary "assertTrue(f.isCancelled());" because the test is not going to reach this line. 
```java
try {
    f.waitForStart();
    fail("Expected an CancellationException to be thrown");
} catch (CancellationException e) {}
```