Update testing instructions to reflect presence of setup button

[jscholes]

Currently, step 2 of the instructions presented to testers by the app reads:

Activate the "Open test page" button below, which opens the example to test in a new window and runs a script that ... <setup script purpose>.

For instance:

Activate the "Open test page" button below, which opens the example to test in a new window and runs a script that sets focus on a link before the grid.

This information hasn't been accurate since around October of 2021, when a button was added to test case pages allowing the setup script to be manually executed. As such, I suggest something like the following wording instead:

2. Activate the "Open test page" button, which opens the example to test in a new window and sets focus on the "Run Test Setup" button.

3. Press the "Run Test Setup" button, which runs a script that <setup script description>, allowing testing commands to be executed from that point. You should press this button again whenever you reopen or refresh the example.

[jscholes]

CC @richnoah / @s3ththompson

[mcking65]

This is covered by w3c/aria-at#977. Currently the details are in w3c/aria-at#978, but they will be integrated into the 977 spec.