tests: don't run test-gpg if not in a working directory

test-gpg has to be run in a mercurial working directory as it uses
that to verify that it hasn't modified the trustdb.gpg file.  Skip the
test if it is running in an exploded tarball.

tests/test-gpg.t

@@ -1,31 +1,35
 Test the GPG extension
 
   $ "$TESTDIR/hghave" gpg || exit 80
+  $ if ! hg identify -q > /dev/null; then
+  >     echo "skipped: not a Mercurial working dir" >&2
+  >     exit 80
+  > fi
   $ cat <<EOF >> $HGRCPATH
   > [extensions]
   > gpg=
   > 
   > [gpg]
   > cmd=gpg --no-permission-warning --no-secmem-warning --no-auto-check-trustdb --homedir $TESTDIR/gpg
   > EOF
   $ hg init r
   $ cd r
   $ echo foo > foo
   $ hg ci -Amfoo
   adding foo
 
   $ hg sigs
 
   $ hg sign 0
   Signing 0:e63c23eaa88a
 
   $ hg sigs
   hgtest                             0:e63c23eaa88ae77967edcf4ea194d31167c478b0
 
   $ hg sigcheck 0
   e63c23eaa88a is signed by:
    hgtest
 
 verify that this test has not modified the trustdb.gpg file back in
 the main hg working dir
   $ hg --cwd $TESTDIR status gpg