tests: Explicitly build target 'all' by default

tests: Explicitly build target 'all' by default

Commit 68f40643 (Build tests by default) added new targets in our
top-level Makefile, that caused us to effectively run
'cd tests && make' as part of the default build. Since no explicit
target is provided, 'make' tries to build the first target in the
given Makefile. On some platforms (such as *BSD), 'make' finds the
first defined target as a pattern rule (%.c) from our included
makefiles, and tries to build the target %.c, which it cannot do. This
causes the build to fail with:

    cd tests && make
    make[3]: don't know how to make %.c. Stop

To fix this, just explicitly build the 'all' target when we build our
tests by default.

Change-Id: I319271482685ec35087c470d95fdcaec6e1d8c47
Reviewed-on: https://gerrit.openafs.org/13993
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>