travis: Make check-acceptance job more verbose

It will help us debug issues when tests fail.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

.travis.yml

@@ -225,7 +225,7 @@ matrix:
     # Acceptance (Functional) tests
     - env:
         - CONFIG="--python=/usr/bin/python3 --target-list=x86_64-softmmu,mips-softmmu,mips64el-softmmu,aarch64-softmmu,arm-softmmu,s390x-softmmu,alpha-softmmu"
-        - TEST_CMD="make check-acceptance"
+        - TEST_CMD="make AVOCADO_SHOW=test check-acceptance"
       addons:
         apt:
           packages: