Debian: Use /bin/bash as the shell for regression test scripts.

Debian: Use /bin/bash as the shell for regression test scripts.

/bin/sh on Debian is a minimal shell called 'dash' which doesn't
support some features we need such as the particular 'function'
syntax used by regressions/test-stringlist.sh, and therefore
this script was failing on Debian.

Change all of these scripts to use #!/bin/bash explicitly to avoid
these sorts of problems.