tests: Pass the flat_namespace option to the linker

tests: Pass the flat_namespace option to the linker

This fixes vircryptotest on macOS 12 (Monterey).

The test relies on library injection (using DYLD_INSERT_LIBRARIES)
to replace the normal random functions with functions giving predictable
results, defined in virrandommock.c. However, using DYLD_INSERT_LIBRARIES
only works when building with flat namespaces.

Adding the -Wl,-flat_namespace option to the linker fixes the problem.
The option was already defined in the top-level meson.build, but had been
forgotten in the test linker arguments.

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>