test suite: gnupg: Save stdin on apt-key's import invocations

Recognise these with an ad-hoc pattern on the command line arguments.
When they occur, save stdin to a temporary file.

We can't do this generally, because stdin might be some thing that
gpg's caller doesn't expect gpg to eat.

When we do this saving, print the size in bytes of the input file.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>