Add symlink to the ct_run of the last commot test run

[pablocostass]

Implements #1734. In case of not being able to create the
symlink rebar3 warns through ?DEBUG.

[pablocostass]

Oh damn, I used string:find/2 but it was added in OTP 20. In earlier versions I should use string:rstr/2 I guess. Are we okay with conditional functions (depending on the OTP version) @ferd?

[ferd]

@pablocostass see rebar_string for a module where we keep compatibility; the tricky bit is to get an equivalent algorithm pre-20 just without unicode support; that might be frustrating. Otherwise, you can always use the re module (with the unicode option in most cases) to do your lookups in a compatible way through regexes; we do cheat a lot with that one.

[ferd]

Small gotcha, this doesn't seem to get triggered when the tests fail, which is arguably when I want to look at the logs the most.