Merge #15022: tests: Upgrade Travis OS to Xenial

b6f0db69a9 Increase timeout of featuer_assumevalid test to fix flaky tests (Graham Krizek)
aa9aca85f1 If tests are ran with (ASan + LSan), Docker needs access to ptrace (Graham Krizek)
a3b8b43663 Update Travis base OS to Xenial (Graham Krizek)

Pull request description:

  Update base Travis OS to `xenial` from `trusty`.

  Link to Travis Docs for Xenial: https://docs.travis-ci.com/user/reference/xenial/

  As noted in the documentation, Docker version is also updated from `17.06` to `18.06`

  Also includes:
      - If running Bitcoin config with LSan sanitizer, Allow ptrace in Docker run command
      - Increase timeout of feature_assumevalid test to fix flaky tests

Tree-SHA512: baf2eda0cbb9990c43c76de1aebc8dd4a3f540323ac1fe2e164ac3bcf1fe3afa3e5b026bfeb5d650dae09a6854695d5744c1130c2fa82ece86c6835ba152f68d

test/functional/feature_assumevalid.py

@@ -180,7 +180,7 @@ class AssumeValidTest(BitcoinTestFramework):
         for i in range(2202):
             p2p1.send_message(msg_block(self.blocks[i]))
         # Syncing 2200 blocks can take a while on slow systems. Give it plenty of time to sync.
-        p2p1.sync_with_ping(120)
+        p2p1.sync_with_ping(150)
         assert_equal(self.nodes[1].getblock(self.nodes[1].getbestblockhash())['height'], 2202)
 
         # Send blocks to node2. Block 102 will be rejected.