Fix reading of files with Mac EOLs in wxTextFile.

The last CR-terminated line wasn't handled correctly.

Fix this now and add unit tests to ensure that it stays fixed.

Closes #15583.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75009 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Simplify the code in wxTextFile::OnRead() slightly.

No real changes, just use a self-explanatory "lastWasCR" variable instead of
storing the last character read which wasn't used for anything except
comparing it with '\r'.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75010 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775