selftests: net: fix spelling and grammar mistakes

Fix several spelling and grammatical mistakes in output messages from
the net selftests to improve readability.

Only the message strings for the test output have been modified. No
changes to the functional logic of the tests have been made.

Signed-off-by: Praveen Balakrishnan <praveen.balakrishnan@magd.ox.ac.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250828211100.51019-1-praveen.balakrishnan@magd.ox.ac.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>