test_anonymous-coalescing test that a non-anonymous h2 connection can be reused for a anonymous request and vice versa. The connection will be reuse only if it experienced. The connection is experience if a non-null transaction is added to the connection when tls handshake was been finished. The test use only one transaction to make a connection "experienced" but this is racy and may not mark the connection "experienced":

• if a null transaction is used to drive tls handshake, the non-null transaction will be added after the handshake is finished and it will mark the connection "experienced".
• if a non-null transaction is used to drive tls handshake, the connection will not be marked "experienced" and it will be only marked "experience when another transaction is added"

To make sure the connection is marked "experienced" we will always performe 2 transaction (in one case anonymous and in the other non-anonymous) before we try a transaction with reverted anonymous flag.

https://hg.mozilla.org/mozilla-central/rev/597370891189