Fix failing TextFileTestCase::ReadMixedWithFuzzing().

The test failed with 33% probability because it didn't account for
trailing non-newline character.  Fixed and also changed the test to
repeat itself a hundred times, to increase the probability of catching
problems like this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75824 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775