testsuite/Makefile: refactor the -promote ocamltest logic
testsuite/Makefile: refactor the -promote ocamltest logic

The existing approach requires to set all ocamltest flags at once on
recursive calls. This is very inconvenient if we want to set more than
one flag -- we wish to support choosing both -promote
and -keep-test-dir-on-success.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml