test/lazy-omap-stats: overload get_output() to accept both lvalue and rvalue refs

test/lazy-omap-stats: overload get_output() to accept both lvalue and rvalue refs

PR #65014 added std::move() inside the while loop, which required
recreating the command string on each iteration.

Add const std::string& overload that forwards to the existing
std::string&& version, allowing the command to be declared outside
the loop for better efficiency.

Fixes: https://tracker.ceph.com/issues/73838
Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>