tests/shell: Improve fix in sets/0036add_set_element_expiration_0

tests/shell: Improve fix in sets/0036add_set_element_expiration_0

Explicitly eliminate the newgen message from output instead of just the
last line to make sure no other output is dropped by accident. This also
allows the test to pass in unpatched kernels which do not emit the
newgen message despite NLM_F_ECHO if no netlink listeners are present.

Fixes: 46b54fdcf266d ("Revert "monitor: do not print generation ID with --echo"")
Signed-off-by: Phil Sutter <phil@nwl.cc>