libxl: events: Makefile builds internal unit tests

libxl: events: Makefile builds internal unit tests

We provide a new LIBXL_TESTS facility in the Makefile.
Also provide some helpful common routines for unit tests to use.

We don't want to put the weird test case entrypoints and the weird
test case code in the main libxl.so library.  Symbol hiding prevents
us from simply directly linking the libxl_test_FOO.o in later.  So
instead we provide a special library libxenlight_test.so which is used
only locally.

There are not yet any test cases defined; that will come in the next
patch.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>