Query count for some mail tests is not always the same for a given test.
This leads notably to runbot being randomly red although there is no real
error. This commit lessen the issue of having red runbot or wrongly
incremented tests by using the recently-introduced margin parameter on
performance tests.
Some work is still ongoing to find why some tests are not completely
reproducible. In the meantime let us have some warnings instead of errors.
Thibault Delavallée
81a0852077