[app][tests] add a preemption test
[app][tests] add a preemption test

-run 5 threads in parallel, each counting a fixed amount.
If preemption is working, they should all complete in roughly the same time.

GCrean/lk,nvll/lk,hollanderic/lkstuff,sndnvaps/lk,minglun-tsai/lk,ErikCorryGoogle/lk,ianloic/lk,minglun-tsai/lk,ErikCorryGoogle/lk,skabet/lk,pablomarx/lk,srodrig1/lk,M1cha/lk,littlekernel/lk,travisg/lk,grub4android/lk,grub4android/lk,hollanderic/lkstuff,GCrean/lk,srodrig1/lk,grub4android/lk,skabet/lk,srodrig1/lk,grub4android/lk,ErikCorryGoogle/lk,M1cha/lk,grub4android/lk,M1cha/lk,nvll/lk,skabet/lk,sndnvaps/lk,ianloic/lk,ianloic/lk,ianloic/lk,nvll/lk,GCrean/lk,nvll/lk,GCrean/lk,travisg/lk,skabet/lk,pablomarx/lk,M1cha/lk,sndnvaps/lk,M1cha/lk,hollanderic/lkstuff,travisg/lk,minglun-tsai/lk,sndnvaps/lk,ErikCorryGoogle/lk,nvll/lk,GCrean/lk,skabet/lk,travisg/lk,minglun-tsai/lk,littlekernel/lk,pablomarx/lk,littlekernel/lk,pablomarx/lk,littlekernel/lk,pablomarx/lk,sndnvaps/lk,srodrig1/lk,hollanderic/lkstuff,srodrig1/lk