tests/xenstore: Rework Makefile

tests/xenstore: Rework Makefile

In particular, fill in the install/uninstall rules so this test can be
packaged to be automated sensibly.

This causes the code to be noticed by CI, which objects as follows:

  test-xenstore.c: In function 'main':
  test-xenstore.c:486:5: error: ignoring return value of 'asprintf', declared
  with attribute warn_unused_result [-Werror=unused-result]
       asprintf(&path, "%s/%u", TEST_PATH, getpid());
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Address the CI failure by checking the asprintf() return value and exiting.

Rename xs-test to test-xenstore to be consistent with other tests.  Honour
APPEND_FLAGS too.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>