Squish Tests: Can switch to custom instrument

As a developer I would like to verify the following behaviour using Squish:

- Switch to a custom instrument by typing in some fake name and then giving some fake PV prefix.
- Verify that the switch was successful.
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- Switch to a custom instrument by typing in some fake name and then giving a working PV prefix. (Use the current prefix to make the test consistent)
- Verify that the switch was successful.

#### Additional info:
To get/verify the prefix we can use the `Help` -> `About` dialog and the `Client PV Prefix` value.

Start by locating the Squish tests covering instrument switching. Use a fake instrument name with both a fake PV prefix and the current Client PV Prefix from Help → About, then verify that each switch succeeds. Done means both scenarios are automated and pass consistently.

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