Redirect cover module output to .eunit/cover.log

The cover module calls io:format and io:fwrite directly for several types of
informational messages and warnings.  When using meck to mock covered modules
these warnings are triggered and can cause severe polution of the test output.

We can avoid this by starting cover explicitly then setting the group_leader
of that process to a file handle.

src/rebar_eunit.erl

@@ -128,10 +128,13 @@ eunit(Config, AppFile) ->
     Modules = [rebar_utils:beam_to_mod(?EUNIT_DIR, N) || N <- BeamFiles],
     SrcModules = [rebar_utils:erl_to_mod(M) || M <- SrcErls],
 
-    cover_init(Config, BeamFiles),
+    {ok, CoverLog} = cover_init(Config, BeamFiles),
+
     EunitResult = perform_eunit(Config, Modules),
     perform_cover(Config, Modules, SrcModules),
 
+    cover_close(CoverLog),
+
     case EunitResult of
         ok ->
             ok;
@@ -275,9 +278,25 @@ cover_analyze(Config, Modules, SrcModules) ->
             ok
     end.
 
-cover_init(false, _BeamFiles) ->
+cover_close(not_enabled) ->
     ok;
+cover_close(F) ->
+    ok = file:close(F).
+
+cover_init(false, _BeamFiles) ->
+    {ok, not_enabled};
 cover_init(true, BeamFiles) ->
+    %% Attempt to start the cover server, then set it's group leader to
+    %% .eunit/cover.log, so all cover log messages will go there instead of
+    %% to stdout.
+    {_,CoverPid} = cover:start(),
+
+    {ok, F} = file:open(
+                filename:join([?EUNIT_DIR, "cover.log"]),
+                [write]),
+
+    group_leader(F, CoverPid),
+
     %% Make sure any previous runs of cover don't unduly influence
     cover:reset(),
 
@@ -300,7 +319,7 @@ cover_init(true, BeamFiles) ->
                                  [Beam, Desc])
                 end,
             _ = [PrintWarning(Beam, Desc) || {Beam, {error, Desc}} <- Compiled],
-            ok
+            {ok, F}
     end;
 cover_init(Config, BeamFiles) ->
     cover_init(rebar_config:get(Config, cover_enabled, false), BeamFiles).