fix(test): use bash for jobs -r parameter

[bdrung]

Changes

Test 20 - "NFSv3 root=dhcp DHCP IP:path" fails on Debian with following error message in server.log:

jobs -rp
/sbin/init: 105: jobs: Illegal option -r

The jobs parameter -r requires bash.

Checklist

• [x] I have tested it locally
• [ ] I have reviewed and updated any documentation if relevant
• [ ] I am providing new code and test(s) for it

Partially fixes #1901

[bdrung]

Enhanced the commit to also cover TEST-50-MULTINIC which uses jobs -rp as well.