Add test-script for git-tag

These tests are useful to develop the C version for git-tag.sh,
ensuring that the future builtin-tag.c will not break previous
behaviour.

The tests are focused on listing, verifying, deleting and creating
tags, checking always that the correct status value is returned
and everything remains as expected.

In order to verify and create signed tags, a PGP key was also
added, being created this way: gpg --homedir t/t7004 --gen-key
Type DSA and Elgamal, size 2048 bits, no expiration date.
Name and email: C O Mitter <committer@example.com>
No password given, to enable non-interactive operation.

Signed-off-by: Carlos Rica <jasampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>