no license on some test modules

[Jim Meyering]

Hi Bruno,

I'm importing some gnulib modules into a lgpl'd project (libvirt) along
with their tests.  I noticed that several test modules don't specify a
license, which makes gnulib-tool --lgpl fail.

alloca-opt-tests is one example.

I can see a couple ways of fixing it, assuming you are game:
  - allow gnulib-tool to exempt test modules
  - add a permissive License: line

Of course, including the tests is solely so that we can get
better test coverage, and not to include them in any library.
That wouldn't make sense.  But they would end up being distributed.