rcutorture: Simplify functions.sh include path

Inclusions of 'functions.sh' from 'kvm-test-1-run.sh' and
'kvm-recheck*.sh' use its absolute path.  Because the directory containing
'functions.sh' is already in PATH, the full path is unnecessary.  This
commit therefore simplifies the inclusions to use the short relative path.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>