Adjust gdb.python/py-events.exp for Cygwin/MinGW, no fork

The gdb.python/py-events.exp testcase currently uses fork, and relies
on "set detach-on-fork off".

Cygwin does support the fork syscall, but GDB can't follow forks
there, so "detach-on-fork off" has no effect.  And also, there is no
fork on native Windows, which makes the testcase unusable on MinGW
currently.

I don't see any reason the testcase needs to use fork or multiple
inferiors.  We can replace what those parts were testing by more
focused tests:

 - The clear_objfiles event was tested via following a fork.  Instead,
   test a more directed "file" command.

 - Two-inferior quit was being used to test that gdb.ExitedEvent has
   no "exit_code".  Quitting while an inferior is being debugged makes
   GDB kill the inferior.  What's really being tested is the kill
   path, so write an explicit (single-inferior) kill test.

Tested on x86_64-unknown-linux-gnu.

Change-Id: I21ee8af7b52653c6fdff9b4c1596cdde3cfe751a