Get rid of `invokelatest()` for calling testcase functions

[fjarri]

Since currently the test files are loaded dynamically, we have to call the testcase functions via invokelast(). Theoretically, this leads to low performance. I tried to work around it by creating a file that includes all the required test files statically and running a child process that actually executed the tests; the speed seemed to be the same. Need to revisit this when the test suite is more complicated.