Change error shrinking to only compare the type

This is an improvement in the short term, since this allows errors
with different messages to shrink better. A followup to this
commit will allow users to hook into this a bit better, by
providing a custom comparison function for their errors.

src/teststate.jl

@@ -105,7 +105,7 @@ function test_function(ts::TestState, tc::TestCase)
             old_frame = first(old_trace)
             frame = first(trace)
             # if the error isn't the same, it can't possibly be better
-            if !(err == old_err && frame == old_frame)
+            if !(typeof(err) == typeof(old_err) && frame == old_frame)
                 @warn "Encountered an error, but it was different from the previously seen one - Ignoring!" Error=err Location=frame
                 return (false, false)
             end