meson: tests: make fullmode-with-stun non-parallel and disable fuzzing test

test-fullmode-with-stun needs to run in non-parallel mode to work.

test-pseudotcp-fuzzy sometimes just times out, which needs investigating.

meson test -C builddir --repeat=20
2 jobs for meson-tests in 4 minutes and 24 seconds (queued for 5 minutes and 38 seconds)
Status Job ID Name Coverage
  Build
passed #64026
build autotools

00:01:55

passed #64027
build meson

00:02:29