ct: skip instead of halt on missing/unknown suite

This allows `rebar ct suites=abc` to consider all suites when
you have a rebar setup with multiple sub_dirs.

Previously, rebar halted after it could not find the suite in the
first dir. But the suite might be present in another dir (when
sub_dirs contains multiple dirs).

This commit makes it so instead of halting, it prints a warning and
continues with looking for the suite in the other `sub_dir`s.

Note -- This uses try/catch to cause the test to be skipped because
otherwise I needed to adjust the return values of 4 functions, the
code path is pretty deeply nested here. Otherwise the whole call chain
needed to be adjusted for this return value:

`run_test -> make_cmd -> get_suites -> find_suite_path`

IMHO, I think for exceptional cases like this it is fine to use throw;
specially since only the {skip} is catched and nothing else.

src/rebar_ct.erl

@@ -65,7 +65,12 @@ run_test_if_present(TestDir, LogDir, Config, File) ->
                           ++ " SUITES - skipping\n", [TestDir]),
                     ok;
                 _ ->
-                    run_test(TestDir, LogDir, Config, File)
+                    try
+                        run_test(TestDir, LogDir, Config, File)
+                    catch
+                        throw:skip ->
+                            ok
+                    end
             end
     end.
 
@@ -270,8 +275,10 @@ find_suite_path(Suite, TestDir) ->
     Path = filename:join(TestDir, Suite ++ "_SUITE.erl"),
     case filelib:is_regular(Path) of
         false ->
-            ?ERROR("Suite ~s not found\n", [Suite]),
-            ?FAIL;
+            ?WARN("Suite ~s not found\n", [Suite]),
+            %% Note - this throw is caught in run_test_if_present/3;
+            %% this solution was easier than refactoring the entire module.
+            throw(skip);
         true ->
             Path
     end.