[ci] try running tests in nix-shell

run_tests.sh

@@ -10,11 +10,12 @@ if [ -z "$TEST_SHELLS" ]; then
 fi
 echo "== Will run all tests with following shells: ${TEST_SHELLS}"
 
-cd -P -- "${0%/*}"  # Change to directory this script lives in
+cd -P -- "${0%/*}" # Change to directory this script lives in
 for test in $(find test/lib -name *_test.sh); do
   for shell in $TEST_SHELLS; do
     echo "== Running tests with [$shell]: $test"
     $shell $test || failed=true
+    printf '==\n\n'
   done
 done