Again instead of calling ltl2tgba dozen of times with different options
for various formulas, this implements a single executable that reads
formulas from a file, translate them using the different setups, and
dump statistics for comparison.  Valgrind now only has to be started
once.

* src/tgbatest/checkta.cc: New file.
* src/tgbatest/Makefile.am: Use it.
* src/tgbatest/ltl2ta.test: Rewrite using checkta.
* src/tgbatest/ltl2tgba.cc: Remove a unused variable.