Use OUnit2 from opam and fix deprecation warnings (#5874)

Author: cknitt

dune-project

@@ -26,6 +26,8 @@
    (= 1.6.9))
   (js_of_ocaml-compiler
    (= 4.0.0))
+  (ounit2
+   (= 2.2.6))
   (reanalyze
    (= 2.23.0))
   dune))

jscomp/dune

@@ -1,5 +1,5 @@
 (dirs bsb bsb_exe bsb_helper bsb_helper_exe bsc cmij common core depends ext
-  frontend gentype jsoo js_parser ml napkin ounit ounit_tests outcome_printer
+  frontend gentype jsoo js_parser ml napkin ounit_tests outcome_printer
   super_errors)
 
 (env