Fix Devel::Peek's tests when building with -DPERL_NO_COW

One test had a pattern with a (?:...)? construction split across two lines,
specifically enclosing the newline at the end of a line, and the line that
followed.

When perl is built with -DPERL_NO_COW or -DPERL_OLD_COPY_ON_WRITE the test's
do_test() driver routine would remove the second line, which included the
closing ')?', leaving a an invalid regex. The solution is to remove the
(?: )? completely, as the driver routine's line removal has the same intent
as the (?: )? construction.
Simplify the conditional logic for COW_REFCNT in Devel::Peek's test.

Instead of having a comment on each COW_REFCNT line to skip the line on
earlier versions, move the skip into the existing code which skips the same
lines on current versions built without COW.