[autotest] Improve the pool inventory message.

Based on feedback from a recent deputy, change the pool inventory
message in site_utils/lab_inventory.py to make the expectations
clearer.

BUG=None
TEST=unit tests

Change-Id: I614f7f4a3c0a2095ea4973c1512ebcedae6bde1f
Reviewed-on: https://chromium-review.googlesource.com/275293
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
1 file changed