tools: check more strictly for bash shebang in "check-tree.sh"

tools: check more strictly for bash shebang in "check-tree.sh"

There is no problem in principle to allow any executable/shebang. However,
it also not clear why we would want to use anything except bash. Unless
we have a good use case, check and reject anything else.

Also not that `./tests/shell/run-tests.sh -x` only works if the shebang
is either exactly "#!/bin/bash" or "#!/bin/bash -e". While it probably
could be made work with other shebangs, the simpler thing is to just use
bash consistently.

Just check that they are all bash scripts. If there ever is a use-case,
we can always adjust this check.

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