libc tests: Rename the quick_exit test file, fix style

[markj]

Summary

Call it libc_exit_test instead of exit_test because the NetBSD test
suite already has a file with the latter name. This is in preparation
for adding other exit()-related tests.

[emaste]

Commit message should be "Call it libc_exit_test instead of quick_exit_test", no?

[markj]

In D46175#1266192, @emaste wrote:

Commit message should be "Call it libc_exit_test instead of quick_exit_test", no?

This was already committed, 5132e16e1fd98. Indeed, the commit message is wrong. :(

[emaste]

Oh, I guess I misread the message -- it's explaining why it's renamed from quick_exit_test to libc_exit_test not to exit_test.