tests: Always cleanup gpg-agent when exiting

tests: Always cleanup gpg-agent when exiting

Add `libtest_cleanup_gpg()` to the array of commands to run when
exiting. This provides 2 improvements:

1. You don't need to worry about whether the test will spawn a gpg-agent
   and therefore require adding a call to `libtest_cleanup_gpg()`.

2. All the existing users were calling `libtest_cleanup_gpg()` at the
   end of the script. If there was a failure and the script exited
   early, then it wouldn't cleanup and there may be a stray gpg-agent
   hanging around.

Closes: #1799
Approved by: cgwalters