travis: If the comparison-tool fails, dump the tail of the debug log

The entire debug log would be huge, and could cause issues for automated tools
like travis. Printing 200 lines is an initial guess at a reasonable number,
more may be required.

qa/pull-tester/run-bitcoind-for-test.sh.in

@@ -29,4 +29,8 @@ fi
 kill $BITCOIND && wait $BITCOIND
 
 # timeout returns 124 on timeout, otherwise the return value of the child
+
+# If $RETURN is not 0, the test failed. Dump the tail of the debug log.
+if [ $RETURN -ne 0 ]; then tail -n 200 $DATADIR/regtest/debug.log; fi
+
 exit $RETURN