Check for `__cause__` in Salt unit test when raising exceptions

[TeddyAndrieux]

Component:

'salt', 'tests'

Why this is needed:

Since #3066 we start using raise <exception> from <cause> so in unit test currently we can only check the content of the last exception raised.

What should be done:

It could be interesting to check as well the cause of the last exception

Implementation proposal (strongly recommended):

Create a new assertRaisesSomething function that basically do the exact same as the assertRaisesRegex but also check the content of __cause__ of the exception