meson: Mark 1bit-emufutex test as slow
Emulated futexes are slower than real ones; if they were not, there would be no point in using the real futexes. On some machines they are sufficiently slow to cause test timeouts.
In particular, Debian's i386 autobuilder 'binet' is not particularly fast:
86/254 glib:glib / 1bit-mutex OK 28.29 s 87/254 glib:glib / 1bit-emufutex TIMEOUT 30.02 s
28.29 seconds is also uncomfortably close to the timeout, and lots of Debian's other architectures are slower than i386 and are hitting timeouts, so I'm going to send a separate MR to increase the arbitrary timeouts globally, when I have a better idea of what a good increase would be.