[systemtest] Properly quote executed command
[systemtest] Properly quote executed command

Otherwise the command can't be copy-pasted into a terminal window (with
the same effect as when run through pytest).

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan