Tests don't run on FreeBSD 11.2

[Duncan Bayne]

The developer docs seem out of date:

## Tests

You can start the test suite by running the command `make test`.

I think the Makefile was removed as of commit fa2976b. It used to do this:

run_tests:
       './test/shared-spec.sh'
       './test/stack-spec.sh'

If I run those manually, fixing up shebangs to be FreeBSD compatible, it works:

...
--SUMMARY
9 PASSED
0 FAILED
...
--SUMMARY
18 PASSED
0 FAILED

So I think what needs to happen is:

1. Shell scripts not executable should be marked as such.
2. Shebangs should be changed to be #!/usr/bin/env bash.
3. Makefile should be reintroduced with test target, that runs the above.

@mbailey your thoughts? Happy to raise a PR for this.