This is a follow up to D88792 which found an issue in a call to PExpectTest's expect function
that allows passing a string to the substrs parameter. However this issue was found by just
grepping and TestPExpect's expect function is still accepting a single string as a value to substrs.
This patch adds the same sanity check that D88792 added to the PExpectTest's implementation
of expect and also adds a small test for it.