tests/Makefile: autogenerate list of symbols to be wrapped

tests/Makefile: autogenerate list of symbols to be wrapped

Rather than manually listing all the -Wl,--wrap=... options, autogenerate
a list of functions to be wrapped in a cmocka test. Allowing this not only
for the test file itself but also for additional "test library" source files
requires some Makefile incantations.

Moreover, no need to list globals.c in dependencies, automatic dependency
tracking takes care for this (and avoids recompilation of tests that don't
pull in global.c).

Signed-off-by: Martin Wilck <mwilck@suse.com>