Support testing frameworks

[oubiwann]

There are three testing frameworks that should be supported:

• Erlang's eunit
• Erlang's Common Test
• the LFE ltest framework

Tasks

• [x] Examine each of the above and the most direct, straight-forward means of supporting them in a rebar3 plugin
• [x] Update the ltest library to decrease the number of dependencies and use the new rebar3_lfe plugin (depends upon https://github.com/lfex/ltest/milestone/1)
• [x] Review lfe-rebar3/test plugin and determine how much of that project's code may be reused
• [ ] Add the bare-minimum code needed to support eunit tests written in LFE via rebar3 eunit
• [ ] Add the bare-minimum code needed to support Common Tests written in LFE via rebar3 ct
• [x] Add the bare-minimum code needed to support tests written using ltest via rebar3 lfe ltest
• [ ] Explore the possibility of adding the command rebar3 lfe test which will run all tests in a project written against all supported test frameworks

For CT support, see the following:

• https://github.com/lfe-rebar3/ct (9 yrs old, so probably old rebar)
• https://github.com/erlang/rebar3/blob/master/src/rebar_prv_common_test.erl

[oubiwann]

We should get eunit support for free after compiling -- for both direct use of eunit and use via ltest.

[oubiwann]

I think we'll need rebar3 lfe test to run the test runner from the ltest project, though.