Add a test for verifying procinfo note inside coredumps.

Add a first test for triggering a core dump in the debugged process
(via PT_DUMPCORE) and verifying it.  The test finds procinfo note
and checks its contents.

The core dump is processed through libelf.  However, it only provides
for finding all note segments (or sections?).  I had to implement
finding and processing individual notes myself.  I've added
a core_find_note() function that will be reused in future tests.

Reviewed by kamil.

(mgorny)