run-tests: extract Popen logic to a single method
This removes 3 lines in total LOC and makes the upcoming changes easier.
Differential Revision: https://phab.mercurial-scm.org/D948
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2