[ng] tests: add "spy" test for '.PHONY' support

[ng] tests: add "spy" test for '.PHONY' support

Some make implementations (especially Solaris make) have some problems
in '.PHONY' support, and this has had the consequence that mainstream
Automake hadn't been able to fully take advantage of the '.PHONY'
semantics to simplify some of its idioms or recipes.  Since we now
want to assume and take advantage of such semantics, we add a new "spy"
test case verifying that they truly hold GNU make, to avoid unwelcome
surprises.

* t/spy-phony.sh: New test.
* t/list-of-tests.mk: Add it.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>