Improve and update the 'etags' test suite

Improve and update the 'etags' test suite

* lib-src/etags.c (mercury_pr): Remove redundant comment.

* test/manual/etags/merc-src/accumulator.m: Add more complex
declarations.
* test/manual/etags/ETAGS.good_1:
* test/manual/etags/ETAGS.good_2:
* test/manual/etags/ETAGS.good_3:
* test/manual/etags/ETAGS.good_4:
* test/manual/etags/ETAGS.good_5:
* test/manual/etags/ETAGS.good_6:
* test/manual/etags/CTAGS.good: Adapt to latest changes in 'etags'
and the test suite.  (Bug#47408)