Add random_suite_order option to eunit command

Option takes either 'true' or a numeric seed value. If true is passed, a
random seed is generated and used. The numeric seed value is for
repeatability.

The idea here is to root out test suites that are order dependant, or
that fail in the presence of certain orderings.

src/rebar_eunit.erl

@@ -150,7 +150,7 @@ run_eunit(Config, CodePath, SrcErls) ->
                         AllBeamFiles),
     OtherBeamFiles = TestBeamFiles --
         [filename:rootname(N) ++ "_tests.beam" || N <- AllBeamFiles],
-    ModuleBeamFiles = BeamFiles ++ OtherBeamFiles,
+    ModuleBeamFiles = randomize_suites(Config, BeamFiles ++ OtherBeamFiles),
 
     %% Get modules to be run in eunit
     AllModules = [rebar_utils:beam_to_mod(?EUNIT_DIR, N) || N <- AllBeamFiles],
@@ -233,6 +233,33 @@ get_suites(Config) ->
             Suites
     end.
 
+%%
+%% == randomize suites ==
+%%
+
+randomize_suites(Config, Modules) ->
+    case rebar_config:get_global(Config, random_suite_order, undefined) of
+        undefined ->
+            Modules;
+        "true" ->
+            Seed = crypto:rand_uniform(1, 65535),
+            randomize_suites1(Modules, Seed);
+        String ->
+            try list_to_integer(String) of
+                Seed ->
+                    randomize_suites1(Modules, Seed)
+            catch
+                error:badarg ->
+                    ?ERROR("Bad random seed provided: ~p~n", [String]),
+                    ?FAIL
+            end
+    end.
+
+randomize_suites1(Modules, Seed) ->
+    random:seed(35, Seed, 1337),
+    ?CONSOLE("Randomizing suite order with seed ~b~n", [Seed]),
+    [X||{_,X} <- lists:sort([{random:uniform(), M} || M <- Modules])].
+
 %%
 %% == get matching tests ==
 %%