improve coverage of LaTeX/utf8 printers for SERE

* bin/common_output.cc, bin/common_output.hh,
bin/randltl.cc: Adjust so that running "randltl -S" use
the SERE flavor of the spot/latex/utf8 formula printers.
* tests/core/latex.test, tests/core/utf8.test,
tests/python/ltlparse.py: Add more test cases.

bin/randltl.cc

@@ -182,6 +182,7 @@ parse_opt(int key, char* arg, struct argp_state* as)
       break;
     case 'S':
       output = spot::randltlgenerator::SERE;
+      output_ratexp = true;
       break;
     case OPT_BOOLEAN_PRIORITIES:
       opt_pB = arg;