[lmi] Customizing warning handling in tests?

[Vadim Zeitlin]

 Hello,

 Now that the code parsing tables can generate warnings, I'd like to test
that the expected warnings are generated and, also, ensure the warnings are
not actually output to standard output while the test is running to avoid
confusion.

 Unfortunately it doesn't seem possible to customize the warning alert
function to do what I want, i.e. to collect the warnings in a string in
order to compare them to the expected ones later. This looks like a very
unfortunate limitation to me, why have all this flexible alert functions
mechanism only to not allow changing them?

 I can work around this by redirecting std::cout instead using the usual
rdbuf() trick, but this seems like a wrong level for solving this problem.
Could we add a possibility to set up a custom warning alert function
instead?

 Thanks,
VZ