debuginfo.c: Add more hooks for lazy loading debuginfo

debuginfo.c: Add more hooks for lazy loading debuginfo

Add calls to di_load_di in search_one_{symtab,loctab,cfitab} to ensure
that debuginfo is always loaded before searching these tables.

This fixes a bug introduced in commit 60f7e89ba32 that results in
callgrind failing to read source filenames from debuginfo.

Also add a test which checks that callgrind successfully finds a
filename from debuginfo.

Co-Authored-By: Philippe Waroquiers <philippe.waroquiers@skynet.be>