Avoid \Z negative pattern in test exclude list

In RHEL10, the grep version is bumped from 3.6 to 3.11, and grep
no longer recognizes the \Z character anymore.

We have 2 solutions: We can either choose to use `grep -P` to
continue using it, or, alternatively, we can choose a different
`null` match to have an effectively empty exclude list.

The latter seems easy enough: By default, we can just exclude
empty lines ("^$") obtaining the exact same behavior as before.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/run-tests.sh

@@ -301,7 +301,7 @@ fi
 # include everything by default
 test -z "$T_INCLUDE" && T_INCLUDE="-e '.*'"
 # (quickly) exclude nothing by default
-test -z "$T_EXCLUDE" && T_EXCLUDE="-e '\Zx'"
+test -z "$T_EXCLUDE" && T_EXCLUDE="-e '^$'"
 
 # eval to strip re ticks but not expand
 tests=$(grep -v "^#" $T_SEQUENCE |