This was a regression test added for a 'git' command line
used for build stamping. Unfortunately, 'gpg' has proved to
be extremely fragile:
* In recent versions, it appears to always require 'gpg-agent' to be
installed for anything involving secret keys, but for some reason is
not normally marked as requiring gpg-agent in Debian's package
manager.
* It tries to create a Unix domain socket in a subdirectory of $TMPDIR
without checking the path length, which fails when $TMPDIR is too
long to fit in the 'sun_path' field of a sockaddr_un struct (which
typically tops out somewhere between 92 and 108 bytes).
We could theoretically address those by artificially reducing the
script's TMPDIR length and checking for gpg-agent in addition to gpg,
but arguably those should both be fixed upstream instead. On balance,
the incremental value that this test provides does not seem worth the
complexity of dealing with such a fragile third-party tool.