travis: retry tests before giving up

.travis.yml

@@ -58,6 +58,6 @@ script:
     - cd bitcoin-$HOST
     - ./configure --cache-file=../config.cache $BITCOIN_CONFIG_ALL $BITCOIN_CONFIG || ( cat config.log && false)
     - make $MAKEJOBS $GOAL || ( echo "Build failure. Verbose build follows." && make $GOAL V=1 ; false )
-    - if [ "$RUN_TESTS" = "true" ]; then make check; fi
+    - if [ "$RUN_TESTS" = "true" ]; then travis_retry make check; fi
 after_script:
     - if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then (echo "Upload goes here. Something like: scp -r $BASE_OUTDIR server" || echo "upload failed"); fi