Add more tests for subpattern references.

These tests were taken from PCRE's testdata/testinput2 with slight
modifications.
Add some more tests for subpattern references.

These were taken from PCRE's testdata/testinput2 with slight modifications.
Remove FIXME comment from CONVERT-COMPOUND-PARSE-TREE method on :SUBPATTERN-REFERENCE.
Rename REFERENCED-REGISTER-MATCHERS -> REGISTER-MATCHERS.
Reformat comments in the style of other comments in the package.

(No periods, no capitalized sentences.)
Reformat comments added in changes adding subpattern references to 70 columns.