Revert "drop `eunit_first_files' and `eunit_compile_opts'. that's what"

This reverts commit dde60d491f.

rebar.config.sample

@@ -157,6 +157,10 @@
 {eunit_tests, [{application, rebar3}]}
 %% Options for eunit:test(Tests, Opts)
 {eunit_opts, [verbose]}.
+%% Additional compile options for eunit. erl_opts is also used
+{eunit_compile_opts, [{d, some_define}]}.
+%% {erl_first_files, ...} but for Eunit
+{eunit_first_files, ["test/test_behaviour.erl"]}.
 
 
 %% == Overrides ==

src/rebar_prv_eunit.erl

@@ -31,7 +31,7 @@ init(State) ->
                                  {opts, eunit_opts(State)},
                                  {profiles, [test]}]),
     State1 = rebar_state:add_provider(State, Provider),
-    State2 = rebar_state:add_to_profile(State1, test, test_state()),
+    State2 = rebar_state:add_to_profile(State1, test, test_state(State1)),
     {ok, State2}.
 
 -spec do(rebar_state:t()) -> {ok, rebar_state:t()} | {error, string()}.
@@ -89,7 +89,28 @@ format_error({error, Error}) ->
 %% Internal functions
 %% ===================================================================
 
-test_state() -> [{extra_src_dirs, ["test"]}, {erl_opts, [{d, 'TEST'}]}].
+test_state(State) ->
+    ErlOpts = rebar_state:get(State, eunit_compile_opts, []),
+    TestOpts = safe_define_test_macro(ErlOpts),
+    TestDir = [{extra_src_dirs, ["test"]}],
+    first_files(State) ++ [{erl_opts, TestOpts ++ TestDir}].
+
+safe_define_test_macro(Opts) ->
+    %% defining a compile macro twice results in an exception so
+    %% make sure 'TEST' is only defined once
+    case test_defined(Opts) of
+       true -> Opts;
+       false -> [{d, 'TEST'}] ++ Opts
+    end.
+
+test_defined([{d, 'TEST'}|_]) -> true;
+test_defined([{d, 'TEST', true}|_]) -> true;
+test_defined([_|Rest]) -> test_defined(Rest);
+test_defined([]) -> false.
+
+first_files(State) ->
+    EUnitFirst = rebar_state:get(State, eunit_first_files, []),
+    [{erl_first_files, EUnitFirst}].
 
 prepare_tests(State) ->
     {RawOpts, _} = rebar_state:command_parsed_args(State),