VTest: show mismatched values when rune length isn't correct

[cohix]

When I'm using AssertBodyString, I can get this error: body length mismatch: got 19 runes, want 18 runes

In this case I would like to know what the expected and actual values were so that I don't need to go adding extra prints to see what went wrong (and this likely applies to other Assert* things as well.