Allow running script from outside the script directory

Author: zachborboa

tests/skip_slow_tests.sh

@@ -1,3 +1,6 @@
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "${SCRIPT_DIR}"
+
 # Let tests know we should skip slow tests.
 export PHP_CURL_CLASS_SKIP_SLOW_TESTS="1"