[PATCH] tests: add compatibility layer

[David Bremner]

mp39590 at gmail.com writes:

> From: Mikhail <mp39590 at gmail.com>
>
> Make test-lib-common.sh load test-lib-<$PLATFORM>.sh to create
> additional shim for platform specifics.
>
> Use test-lib-FREEBSD.sh to call GNU utilities instead of native ones.

We had quite a bit of discussion of this on IRC. My (subjective) summary
is that

- we are generally fine with test-lib-${PLATFORM}.sh
- we prefer functions to aliases (fixed in the latest version)
- some people are uncomfortable with mandating gnu coreutils to run the
  tests on FreeBSD

With respect to the last point, I agree it's not ideal, but it is better
than not running them at all. We can incrementally remove the dependence
on coreutils later if someone is motivated. This solution has the
advantage of being easy, and not imposing a burden on people not using
that platform.

The only thing that still bugs me about this is the variable
BSD_GDB. I'm a bit surprised that the same approach used for coreutils
(i.e. rely on PATH) does not work. I guess because the names are the
same it's less reliable for gdb.  If we do a seperate variable, I'd
prefer to use NOTMUCH_GDB (in line with NOTMUCH_PYTHON) since BSD_GDB
suggests the native one.  This could (eventually) be computed by
configure, but this need not block this patch.

Happy New-Year-In-Some-Timezones,

David