test(assert): drop jq from flaky custom-assertion test

The 'custom assertion with fail shows correct test name' test was the
only one of its three siblings to shell out to jq, and the only one that
flaked on the macOS CI runner (empty/mismatched captured output) while
the two pure-bash siblings never did. Route the failure through
bashunit::fail with a pure-bash json check so the test no longer depends
on an external binary.

Author: Chemaclass

tests/unit/custom_assertions_test.sh

@@ -1,15 +1,19 @@
 #!/usr/bin/env bash
 
-# Custom assertion that uses fail internally
+# Custom assertion that uses fail internally.
+# jq-free on purpose: this scaffolding only needs to route a failure through
+# bashunit::fail deterministically; depending on the external jq binary made the
+# test flaky on the macOS CI runner while the pure-bash sibling tests never were.
 function _assert_valid_json() {
   local json="$1"
 
-  if ! echo "$json" | jq . >/dev/null 2>&1; then
+  case "$json" in
+  '{'*'}' | '['*']') bashunit::state::add_assertions_passed ;;
+  *)
     bashunit::fail "Invalid json: $json"
     return
-  fi
-
-  bashunit::state::add_assertions_passed
+    ;;
+  esac
 }
 
 # Custom assertion that uses bashunit::assertion_failed