Instead of the prior work-around.
Makefile target `internal-prepare-exec-env` was modified to create
`exec_env/testrun` and depend on `dotnetlibs/corerun` (on which it
already depended).
- tests.mk includes all test related code except run-test target
- run-test moved after run target
- TEST_SRCS uses wildcard to collect files
- dotnetlibs/xunit* uses wildcard to copy dependencies for monad-ext
- not %.dll because of the soon to be used xunit.console.netcore.exe
Peter Honeder