servlet: fix threadingTest and update lincheck

[panchenko]

Seems like previously it was not testing all the flows but only: the write/flush calls are added to the queue by runOrBuffer because readyAndDrained is initially false.

So,

• isReady() is never called
• isReadyReturnedFalse never set to true
• maybeOnWritePossible() does nothing

All that makes me think that #9917 is caused by some bugs in older versions of llincheck, can be closed now.

A more real simulation happens if calling onWritePossible first via initialOnWritePossible, and then it has found a race condition in AsyncServletOutputStreamWriterConcurrencyTest.isReady(), if maybeOnWritePossible() is executed before return isReady.

Additionally updated lincheck,

• it does not need jvmArgs now
• is compatible with java 8 again
• changed asserts from Truth to junit, as lincheck was trying to use com.google.common.truth.Subject.equals(Object)

It still does not detect #12268, but might be needs more time.