injection_points: Re-enable permutation in isolation test "basic"

injection_points: Re-enable permutation in isolation test "basic"

This test has been disabled in 9f00edc22888 due to an instable expected
output, where it would be possible for the wait step to report its
result after the detach step is done.  The expected output was ordered
so as the detach would always report last.

Isolation test permutations have the option to use markers to control
the ordering for cases like this one, as documented in
src/test/isolation/README.  The permutation is enabled once again, this
time with a marker added so as the detach step reports only once the
wait step has finished, ensuring a correct output ordering.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Z6MBZTX5EqQ6A8Zc@paquier.xyz