LU-14653 tests: Correct include path for sanity-lnet test_300

LU-14653 tests: Correct include path for sanity-lnet test_300

We need to supply an appropriate include path for sanity-lnet
test_300 when we're running in tree.

Test-Parameters: trivial testlist=sanity-lnet env=ONLY=300
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Ia04a713ef6f1989507a77a618328d31f74d48e0d
Reviewed-on: https://review.whamcloud.com/43500
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>