selftests/net: Find nettest in current directory

The `nettest` binary, built from `selftests/net/nettest.c`,
was expected to be found in the path during test execution of
`fcnal-test.sh` and `pmtu.sh`, leading to tests getting
skipped when the binary is not installed in the system, as can
be seen in these logs found in the wild [1]:

# TEST: vti4: PMTU exceptions [SKIP]
[ 350.600250] IPv6: ADDRCONF(NETDEV_CHANGE): veth_b: link becomes ready
[ 350.607421] IPv6: ADDRCONF(NETDEV_CHANGE): veth_a: link becomes ready
# 'nettest' command not found; skipping tests
# xfrm6udp not supported
# TEST: vti6: PMTU exceptions (ESP-in-UDP) [SKIP]
[ 351.605102] IPv6: ADDRCONF(NETDEV_CHANGE): veth_b: link becomes ready
[ 351.612243] IPv6: ADDRCONF(NETDEV_CHANGE): veth_a: link becomes ready
# 'nettest' command not found; skipping tests
# xfrm4udp not supported

The `unicast_extensions.sh` tests also rely on `nettest`, but
it runs fine there because it looks for the binary in the
current working directory [2]:

The same mechanism that works for the Unicast extensions tests
is here copied over to the PMTU and functional tests.

[1] https://lkft.validation.linaro.org/scheduler/job/5839508#L6221
[2] https://lkft.validation.linaro.org/scheduler/job/5839508#L7958

Signed-off-by: Daniel Díaz <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/fcnal-test.sh

···
 	TESTS="$TESTS_IPV6"
 fi
 
-which nettest >/dev/null
-if [ $? -ne 0 ]; then
-	echo "'nettest' command not found; skipping tests"
-	exit $ksft_skip
+# nettest can be run from PATH or from same directory as this selftest
+if ! which nettest >/dev/null; then
+	PATH=$PWD:$PATH
+	if ! which nettest >/dev/null; then
+		echo "'nettest' command not found; skipping tests"
+		exit $ksft_skip
+	fi
 fi
 
 declare -i nfail=0

tools/testing/selftests/net/pmtu.sh

···
 }
 
 setup_nettest_xfrm() {
-	which nettest >/dev/null
-	if [ $? -ne 0 ]; then
-		echo "'nettest' command not found; skipping tests"
-	        return 1
+	if ! which nettest >/dev/null; then
+		PATH=$PWD:$PATH
+		if ! which nettest >/dev/null; then
+			echo "'nettest' command not found; skipping tests"
+			return 1
+		fi
 	fi
 
 	[ ${1} -eq 6 ] && proto="-6" || proto=""