add module tests

[asiripanich]

#7 has a testthat test for demography that works on my local machine but doesn't work on CIs. The reason for that is because R CMD check can't seem to find the modules folder when running tests.

> test_check("dymiumModules")
── 1. Error: demography all events (@test-demography.R#8)  ──────────────────────────
Can`t find ../../modules/demography
Backtrace:
 1. modules::use(file.path("..", "..", "modules", "demography"))
 3. modules:::as.module.character(module, reInit = reInit, envir = where)