selftests: net: add helper for checking if nettest is available

A few tests check if nettest exists in the $PATH before adding
$PWD to $PATH and re-checking. They don't discard stderr on
the first check (and nettest is built as part of selftests,
so it's pretty normal for it to not be available in system $PATH).
This leads to output noise:

  which: no nettest in (/home/virtme/tools/fs/bin:/home/virtme/tools/fs/sbin:/home/virtme/tools/fs/usr/bin:/home/virtme/tools/fs/usr/sbin:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin)

Add a common helper for the check which does silence stderr.

There is another small functional change hiding here, because pmtu.sh
and fib_rule_tests.sh used to return from the test case rather than
completely exit. Building nettest is not hard, there should be no need
to maintain the ability to selectively skip cases in its absence.

Reviewed-by: Ido Schimmel <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>
Reviewed-by: Hangbin Liu <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

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