Cmake tests: Simplify test logic for exporting to formats lyx[0-9][0-9]x

Cmake tests: Simplify test logic for exporting to formats lyx[0-9][0-9]x

Since the stop condition is that the last two consecutive created
files are identical, we do not need to check the last file.
Also checking for load of the created files is not needed, because
exporting the previous file implicitly loads too.