Try to use /proc/self instead of /dev/core in the unit test.

/dev/core doesn't seem to exist on the machines running Travis CI builds,
check if /proc/self can work there.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75331 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775