build: make the 'test' target depend on the 'all' target

This would fix some scenarios that some developers could run into:

a) Run `./autogen.sh --enable-tests`, and after that `make test`
without running `make` before (which would yield many "Could not
find file foo.dll" errors).

b) Run `./autogen.sh`, then `make`, then `make test` which would
yield "make[1]: *** No rule to make target `test'.  Stop.", and
(hopefully) make the developer realise that she was lacking the
--enable-tests flag for the configuration phase, so then she'd
then run `./autogen.sh --enable-tests && make test`, which would
cause nunit-console to be run in assemblies which didn't contain
tests.

The (b) case is more severe because it could be very misleading,
making the developer think that tests passed (as the execution of
NUnit runner wouldn't yield any test error), when they may have
not.