[perl #128012] Support mandoc in perl5db.t

# New Ticket Created by  Andreas Voegele 
# Please include the string:  [perl #128012]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=128012 >


When mandoc is installed instead of the traditional man package one of 
the tests in perl5db.t fails with the error message below. With the 
attached patch the output of both man implementations is accepted.

lib/perl5db ...................................................
# Failed test 120 - perldoc command works fine at ../lib/perl5db.t line 2769
#      got 'man: No entry for perlrules in the manual.
# '
# expected /(?^:No manual entry for perlrules)/
FAILED at test 120

• [perl #128012] Support mandoc in perl5db.t by James E Keenan via RT
• [perl #128012] Support mandoc in perl5db.t by Andreas Voegele