`make check-integrationtests` assumes bash is installed

Since commit 78ff65ef, this checks that if you add a test case in
tests/test-*.sh, you don't forget to list it in tests/run-tests.sh.
But it only ran this check if you had bash installed (the process
substitution "<(...)" is a bash-ism).

However `make check-integrationtests` also executes tests/run-tests.sh,
which requires bash (since 49b8a0f1, 4 months earlier than 78ff65ef!).
So we're already assuming that bash is available.