testsuite: fix recentmanager failure on win32

recentmanager tests fail on win32 because g_test_trap_fork isn't supported there.

Replace the deprecated g_test_trap_fork() by g_test_subprocess().

I had to split the "/recent-manager/add" test in 4 parts, seems g_test_subprocess can only be (easily) used once per test.

Also force gtk-recent-files-enabled to TRUE (fixes endless loop waiting for "changed" signal callback)