test: add util.inspect test for null maxStringLength

Add test case to cover currently-uncovered code.

Refs: https://coverage.nodejs.org/coverage-39a7f7663e8f70fc/lib/internal/util/inspect.js.html#L333

PR-URL: #36086
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>

Author: Trott

test/parallel/test-util-inspect.js

@@ -2905,6 +2905,7 @@ assert.strictEqual(
     util.inspect(x, { maxStringLength: 4 }),
     "'aaaa'... 999996 more characters"
   );
+  assert.match(util.inspect(x, { maxStringLength: null }), /a'$/);
 }
 
 {