The script didn't include all the symbols needed by plugins because
libNetworkManager.a, as built by meson, doesn't include symbols from
other static libraries that are linked in. Since we used
libNetworkManager.a to know which symbols are potentiall available
from NM, the result was an incomplete list.

Unfortunately, the only way to include the whole static library is to
create a dependency object and use 'link_whole', but this is only
available in meson >= 0.46. Since 'link_whole' is available for
executables in meson >= 0.40, create a fake executable and use that to
enumerate symbols.

Also add tests to check that plugins can be loaded correctly.

Fixes: dfa2a2b4