tests: add vlib/v/tests/failing_tests_test.v, support `testdata` in `v test-self` too

cmd/tools/vtest-self.v

@@ -123,7 +123,7 @@ fn main() {
 	all_test_files := os.walk_ext(os.join_path(vroot, 'vlib'), '_test.v')
 	testing.eheader(title)
 	mut tsession := testing.new_test_session(cmd_prefix, true)
-	tsession.files << all_test_files
+	tsession.files << all_test_files.filter(!it.contains('testdata' + os.path_separator))
 	tsession.skip_files << skip_test_files
 	mut werror := false
 	mut sanitize_memory := false

vlib/v/tests/failing_tests_test.v

@@ -0,0 +1,21 @@
+import os
+
+fn vroot_path(relpath string) string {
+	return os.real_path(os.join_path(@VMODROOT, relpath))
+}
+
+fn vexecute(relpath string) os.Result {
+	return os.execute('${@VEXE} ' + vroot_path(relpath))
+}
+
+fn testsuite_begin() {
+	os.setenv('VCOLORS', 'never', true)
+}
+
+fn test_returning_options() {
+	res := vexecute('vlib/v/tests/testdata/tests_returning_options_failing_test.v')
+	assert res.exit_code == 1
+	dump(res)
+	assert res.output.contains('tests_returning_options_failing_test.v:13: fn test_example failed propagation with error: failing test with return, err: oh no')
+	assert res.output.contains('tests_returning_options_failing_test.v:19: fn test_example_2 failed propagation with error: oh no')
+}

vlib/v/tests/testdata/tests_returning_options_failing_test.v

@@ -0,0 +1,20 @@
+fn example() ? {
+	return error('oh no')
+}
+
+fn test_simple() {
+	assert true
+	assert true
+}
+
+fn test_example() ? {
+	assert true
+	assert true
+	example() or { return error('failing test with return, err: $err') }
+}
+
+fn test_example_2() ? {
+	assert true
+	assert true
+	example() ?
+}