tests/shell: use bash instead of /bin/sh for tests

tests/shell: use bash instead of /bin/sh for tests

All tests under "tests/shell" are shell scripts with shebang /bin/bash
or /bin/sh. This may seem expected, since these tests are under
"tests/shell" directory, but any executable file would work.

Anyway. The vast majority of the tests has "#!/bin/bash" as shebang.
A few tests had "#!/bin/sh" or "#!/bin/sh -e". Unify this and always use bash.
Since we anyway require bash, this is not a limitation.

Also, if we know that this is a bash script (by parsing the shebang), we
can let the test wrapper pass "-x" to the script. The next commit will
do that, and it is nicer if the shebangs are all uniform.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>