lang: Allow more than one possible error in tests

There are some rare situations with completely symmetrical graphs which
mean that there isn't a "more correct" error. This is due to the
annoying map iteration non-determinism, and so instead of fighting to
remove every bit of that, let's just accept more than one error here.

lang/interpret_test/TestAstFunc2/polymorphic-lambda.txtar

@@ -18,3 +18,4 @@ $out2 = $add($val) # hellohello
 test [fmt.printf("%s + %s is %s", $val, $val, $out2),] {} # simple concat
 -- OUTPUT --
 # err: errUnify: unify error with: topLevel(singleton(func(x) { call:_operator(str("+"), var(x), var(x)) })): type error: str != int
+# err: errUnify: unify error with: topLevel(singleton(func(x) { call:_operator(str("+"), var(x), var(x)) })): type error: int != str

lang/interpret_test/TestAstFunc2/test-monomorphic-bind.txtar

@@ -11,3 +11,4 @@ test "test2" {
 }
 -- OUTPUT --
 # err: errUnify: unify error with: topLevel(singleton(func(x) { var(x) })): type error: str != int
+# err: errUnify: unify error with: topLevel(singleton(func(x) { var(x) })): type error: int != str

lang/interpret_test/TestAstFunc2/test-monomorphic-class-arg.txtar

@@ -13,3 +13,4 @@ class use_polymorphically($id) {
 include use_polymorphically(func($x) {$x})
 -- OUTPUT --
 # err: errUnify: unify error with: topLevel(singleton(func(x) { var(x) })): type error: str != int
+# err: errUnify: unify error with: topLevel(singleton(func(x) { var(x) })): type error: int != str