gdb/dwarf: fix failed assertion in dwarf2_find_containing_comp_unit selftest

gdb/dwarf: fix failed assertion in dwarf2_find_containing_comp_unit selftest

Commit 2f0521c0d6f6 ("gdb/dwarf: fix signature_type created with nullptr
section") added some asserts in the dwarf2_per_cu_data constructor to
verify that the passed dwarf2_per_bfd and dwarf2_section_info are not
nullptr.  However, the dummy dwarf2_per_cu_data objects created in the
dwarf2_find_containing_comp_unit selftests are passed nullptr for those
parameters.

I prefer to keep the asserts in place, as protection for the non-test
code and as self documentation, so fix this by passing some dummy
pointers in the test.

Change-Id: Ic7cdc1b976f7506041b651222234eefc998e473a
Reviewed-By: Tom de Vries <tdevries@suse.de>