BUG: Improved accuracy of scipy.stats.truncexpon

Fixes the bug related on #4035
@ev-br there isnt such class. At line 245 we can se a test for ppf function trunc exception. In our case we have a test for nbinom.pmf. If my test is in wrong place, so that test of line 245, right? Anyway, in my case, sould I add another method them?