test-skeleton: increase default TIMEOUT to 20 seconds

The vast majority of timeouts I've seen w/glibc tests are due to:
 - slow system (e.g. <1 GHz cpu)
 - loaded system (e.g. lots of parallelism)
Even then, I've seen timeouts on system I don't generally consider
slow, or even loaded, and considering TIMEOUT is set to <=10 in ~60
tests (and <=20 in ~75 tests), it seems I'm not alone.  I've just
gotten in the habit of doing `export TIMEOUTFACTOR=10` on all my
setups.

In the edge case where there is a bug in the test and the timeout is
hit, I think we all agree that's either a problem with the test or a
real bug in the library somewhere.  In either case, the incident rate
should be low, so catering to that seems like the wrong trade-off.

Other developers too usually set large timeout factors.  Increase the
default to 20 seconds to match reality.