deduplicate default test set generated by rebar3 eunit

[talentdeficit]

this ONLY attempts to deduplicate test sets that are generated by
rebar in the absence of any user specified tests

this closes #937

[talentdeficit]

the default test set generated by rebar3 is now {application, App} for each App in the project plus any modules in test directories that do not have the same name as any app modules with _tests appended (which will be run as they are covered by {application, App). ideally, it would just include modules that have names that end in _tests but there's been at least one issue opened where the user expected all modules to be included

[ferd]

+1.