tests: Switch one of the Travis jobs to an unsigned char environment (-funsigned-char)

.travis.yml

@@ -116,14 +116,14 @@ jobs:
         CONFIG_SHELL="/bin/dash"
 
     - stage: test
-      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [uses qt5 dev package instead of depends Qt to speed up build and avoid timeout]'
+      name: 'x86_64 Linux  [GOAL: install]  [bionic]  [uses qt5 dev package instead of depends Qt to speed up build and avoid timeout] [unsigned char]'
       env: >-
         HOST=x86_64-unknown-linux-gnu
         PACKAGES="python3-zmq qtbase5-dev qttools5-dev-tools protobuf-compiler libdbus-1-dev libharfbuzz-dev libprotobuf-dev"
         DEP_OPTS="NO_QT=1 NO_UPNP=1 DEBUG=1 ALLOW_HOST_PACKAGES=1"
         TEST_RUNNER_EXTRA="--coverage --extended --exclude feature_dbcrash"  # Run extended tests so that coverage does not fail, but exclude the very slow dbcrash
         GOAL="install"
-        BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports --enable-debug CXXFLAGS=\"-g0 -O2\""
+        BITCOIN_CONFIG="--enable-zmq --with-gui=qt5 --enable-glibc-back-compat --enable-reduce-exports --enable-debug CFLAGS=\"-g0 -O2 -funsigned-char\" CXXFLAGS=\"-g0 -O2 -funsigned-char\""
 
     - stage: test
       name: 'x86_64 Linux  [GOAL: install]  [trusty]  [no functional tests, no depends, only system libs]'