tests: add a proper readme

tests/readme.md

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+# Opam test suite
+
+There is 3 different kind of tests
+* CRAM test in `reftests`. It is used to check opam behaviour. See
+  `reftests/readme.md` for more details.
+* Bench tests in `bench`, using
+  [current-bench](https://github.com/ocurrent/current-bench). It is used to
+  bench some specific operations.
+* OCaml tests in `lib`. For the moment it is used to check some type
+  manipulation and patch code.
+
+There is also other tests:
+* [`opam-rt`](https://github.com/ocaml-opam/opam-rt): historical test suite. it
+  is a mix of opam cli calls and library usage. It is meant to be removed once
+  all test has been ported to `reftests` and `lib`
+* [`opam-crowbar`](https://github.com/ocaml/opam/blob/master/src/crowbar/opam-crowbar.opam):
+  some fuzz testing of some opam library main types.