Merge bitcoin#15022: tests: Upgrade Travis OS to Xenial

b6f0db6 Increase timeout of featuer_assumevalid test to fix flaky tests (Graham Krizek)
aa9aca8 If tests are ran with (ASan + LSan), Docker needs access to ptrace (Graham Krizek)
a3b8b43 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

.travis.yml

@@ -3,7 +3,7 @@
 #   IPv6 support
 
 sudo: required
-dist: bionic
+dist: xenial
 
 os: linux
 language: minimal

.travis/test_04_install.sh

@@ -10,6 +10,8 @@ travis_retry docker pull "$DOCKER_NAME_TAG"
 env | grep -E '^(CCACHE_|WINEDEBUG|LC_ALL|BOOST_TEST_RANDOM|CONFIG_SHELL)' | tee /tmp/env
 if [[ $HOST = *-mingw32 ]]; then
   DOCKER_ADMIN="--cap-add SYS_ADMIN"
+elif [[ $BITCOIN_CONFIG = *--with-sanitizers=*address* ]]; then # If ran with (ASan + LSan), Docker needs access to ptrace (https://github.com/google/sanitizers/issues/764)
+  DOCKER_ADMIN="--cap-add SYS_PTRACE"
 fi
 DOCKER_ID=$(docker run $DOCKER_ADMIN -idt --mount type=bind,src=$TRAVIS_BUILD_DIR,dst=$TRAVIS_BUILD_DIR --mount type=bind,src=$CCACHE_DIR,dst=$CCACHE_DIR -w $TRAVIS_BUILD_DIR --env-file /tmp/env $DOCKER_NAME_TAG)