Change signature of the macro to take the full test name, a separate
description (previously called a label?), and the executable and test
invocation. The overall signature becomes easier.
The rational for the full test name is that a later commit will
introduce the possibility to define physim tests that are checked for
specific times, in which case we need to reuse the test name. Thus,
using the full name from the beginning leads to more clarity, even
though they are a bit longer.
Add an additional safety check that the given simulator is actually a
target defined within the project (in other words, it actually exists).
Update the documentation correspondingly.