Tests: intermittent TestTimedOutException in FilteredBlockAndPartialMerkleTreeTests > serializeDownloadBlockWithWallet

```
org.bitcoinj.core.FilteredBlockAndPartialMerkleTreeTests > serializeDownloadBlockWithWallet[1] FAILED
org.junit.runners.model.TestTimedOutException: test timed out after 15 seconds
at sun.misc.Unsafe.park(Native Method)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:537)
at com.google.common.util.concurrent.FluentFuture$TrustedFuture.get(FluentFuture.java:86)
at com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly(Uninterruptibles.java:201)
at com.google.common.util.concurrent.Futures.getUnchecked(Futures.java:1233)
at org.bitcoinj.core.PeerGroup.start(PeerGroup.java:1076)
at org.bitcoinj.core.FilteredBlockAndPartialMerkleTreeTests.serializeDownloadBlockWithWallet(FilteredBlockAndPartialMerkleTreeTests.java:168)
```

See: https://github.com/bitcoinj/bitcoinj/runs/3515181687?check_suite_focus=true

@schildbach Should we increase the timeout again?

Start with FilteredBlockAndPartialMerkleTreeTests.serializeDownloadBlockWithWallet at line 168 and the PeerGroup.start call in PeerGroup.java:1076. Reproduce the intermittent 15-second timeout, inspect the test's startup and waiting behavior, and determine whether the timeout or the underlying test setup needs adjustment. Done means the test completes reliably with a justified timeout.

Written by the indexing model from the issue text.