Fix broken test runner (!).

If a test expected an error and found at least one, it would not fail
on any other expected errors that didn't occur.

Also, some tests were expecting a compile time error message even though
the test script doesn't validate those (yet).

The test function was getting monolithic, so I went ahead and split it
into a separate little class.

test/limit/variable_name_too_long.wren

@@ -1,2 +1,2 @@
-var i234567890i234567890i234567890i234567890i234567890i2345 = "value" // expect error
+var i234567890i234567890i234567890i234567890i234567890i234567890i2345 = "value" // expect error
 class c234567890c234567890c234567890c234567890c234567890c234567890c2345 {} // expect error