gdb/testsuite/dwarf: fix abbrev section name when putting type unit in DWO file

Fix what looks like a copy paste error resulting in the wrong abbrev
section name.  The resulting section name in my test was
".debug_info.dwo.dwo", when it should have been ".debug_abbrev.dwo".

Change-Id: I82166d8ac6eaf3c3abc15d2d2949d00c31fe79f4
Approved-By: Tom Tromey <tom@tromey.com>