[PATCH, testsuite] Avoid testsuite harness failure on i686-mingw32 host

[Eli Zaretskii]

> From: Maxim Kuvyrkov <maxim@codesourcery.com>
> Date: Fri, 16 Nov 2012 16:50:50 +1300
> 
> This patch improves testing on i686-mingw32 host.  One of the tests in interrupt.exp assumes that Control-D is EOF, which is not the case for Windows hosts.  After the test timeouts the testsuite doesn't get GDB prompt back, which causes all subsequent tests to fail.

Instead of sending Ctrl-C, isn't it better to send Ctrl-Z, which is
the Windows equivalent of the Posix Ctrl-D?