ocamltest: introduce the OCAMLTEST_SKIP_TESTS environment variable
ocamltest: introduce the OCAMLTEST_SKIP_TESTS environment variable

This variable contains a space-separated list of files. It instructs
ocamltest to skip all the tests in these files.

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