tests: consistently source test-lib.sh in scripts

This unifies the test scripts to all use the similar pattern added for
schematests in ace4aecd. This gives the following

- Enables running all tests from outside of tests/ dir
- Drops redundant abs_* definitions, which are set by test-lib.sh
- Drops unnecessary srcdir variable which was only used for sourcing
    test-lib.sh

Behavior changes:

- srcdir can no longer be overwritten, but I don't know why anyone would
    really need to...
- Script VERBOSE setting no longer prints commands executed by test-lib.sh.
    if anyone cares I suggest handling this in test-lib.sh which already
    has other verbose style handling

tests/virsh-read-non-seekable

@@ -17,17 +17,13 @@
 # along with this program.  If not, see
 # <http://www.gnu.org/licenses/>.
 
-test -z "$srcdir" && srcdir=`pwd`
-test -z "$abs_top_srcdir" && abs_top_srcdir=`pwd`/..
-test -z "$abs_top_builddir" && abs_top_builddir=`pwd`/..
+. "$(dirname $0)/test-lib.sh"
 
 if test "$VERBOSE" = yes; then
   set -x
   $abs_top_builddir/tools/virsh --version
 fi
 
-. "$srcdir/test-lib.sh"
-
 fail=0
 
 cat <<\EOF > dom