test: avoid unnecessary extraction of the test fingerprint

FINGERPRINT is already exported by add_gnupg_home, so this is
unnecessary.  This change also happens to get rid of the superfluous
check-trustdb spew from the test suite that looked like this:

gpg: checking the trustdb
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>