Fix issues of naming in rna-transcription.

The following changes all make it easier to run the tests:

- Rename the test sources to match what eunit wants: tests for `foo` live in
  `foo_tests` (note the 's' at the end there).
- Expect the module-under-test to be named `rna_transcription`.
- Modify the test source to explicitly import the solution expected from the user.
- Add instructions for compilation and running.

All of this follows the lead of the previous exercise, `leap`.

Signed-off-by: Magnus Therning <magnus@therning.org>