This test supposed to check the test base we are using for pexpect tests, but instead it used the normal TestBase
class we use for all other tests. TestBase already had the substrs type check since D88792 so this
test was passing because of that.
This just changes the test base of the test to the pexpect one so that the expect calls find their intended
target function. Also moves the check to the very start so that we can check the argument without
actually having to start a terminal and all that jazz.
(I found this by accident as D88792 got somehow reverted in a downstream branch so this test started