Dump interactive logs on travis test failure

As far as I know we can't access the build artifacts from Travis, so we
can't check the interactive logs after a test failure. Add an
environment variable that causes the test runner to dump the logs
itself, and set that variable for Travis.

tests/interactive.fish

@@ -42,6 +42,12 @@ for i in *.expect
         # clean up tmp files
         rm -f $i.tmp.{err,out,log}
     else
+        if set -qgx SHOW_INTERACTIVE_LOG
+            # dump the interactive log
+            # primarily for use in travis where we can't check it manually
+            echo "Log for file $i:"
+            cat $i.tmp.log
+        end
         echo "File $i failed tests"
     end
 end