Skip slow tests on Travis

Closes GH-12697

travis/test.sh

@@ -4,6 +4,7 @@ set -ex
 # ARM64 CI reports nproc=32, which is excessive.
 if [ -z "$ARM64" ]; then export JOBS=$(nproc); else export JOBS=16; fi
 
+export SKIP_SLOW_TESTS=1
 export SKIP_IO_CAPTURE_TESTS=1
 ./sapi/cli/php run-tests.php -P \
     -g "FAIL,XFAIL,BORK,WARN,LEAK,SKIP" --offline --show-diff --show-slow 1000 \