[perl #43349] [TODO] config/auto/gdbm.pm: Write unit tests

[James Keenan via RT]

Much of the guts of this step class's runstep() routine have been
refactored out into internal subs and methods in order to increase their
testability.  4 test files have been added.  Statement coverage is
satisfactory, given that the remaining uncovered statements are of the
uncoverable, eval { $conf->cc_build } variety.  Resolving ticket.