Automate EC/ES tests

This came up one again in https://github.com/trixi-framework/Trixi.jl/pull/900#discussion_r725094522. If someone wants to earn a T-shirt, this would be a nice Hacktoberfest project. The idea would be to add an optional keyword argument `test_ec` to `@test_trixi_include` with default value `false`. If `test_ec` is set to `true` (or a non-zero tolerance), we should perform an automatic check whether semidiscrete entropy-conservation (EC) holds. It would probably be sufficient to just test the final numerical solution.

Start by locating the definition and uses of @test_trixi_include, then read the linked discussion in PR #900 for the intended EC check. The work is done when the optional test_ec argument defaults to false and, when enabled with a true value or non-zero tolerance, checks the final numerical solution for semidiscrete entropy conservation.

Written by the indexing model from the issue text.