[#1128] Refactor test framework for diagnostics

[robertoaloi]

Description

After this cleanup and refactoring, a typical diagnostics suite will look like:

-spec escript_warnings(config()) -> ok.
escript_warnings(_Config) ->
  Path = src_path("diagnostics_warnings.escript"),
  Source = <<"Compiler">>,
  Errors = [],
  Warnings = [ #{ code => <<"L1230">>
                , message => <<"function unused/0 is unused">>
                , range => {{23, 0}, {24, 0}}
                }
             ],
  Hints = [],
  els_test:run_diagnostics_test(Path, Source, Errors, Warnings, Hints).

Fixes #1128 .