Delete test target descriptions when exiting

Delete test target descriptions when exiting

Looking at the address sanitizer output, this was a quite low hanging
fruit.  We create target_desc objects for testing that we never free.
Saving them in unique_ptrs takes care of it.

I created a small struct to hold these because I thought it would help
readability.

gdb/ChangeLog:

	* target-descriptions.c (struct xml_test_tdesc): New.
	(xml_tdesc): Change type to std::vector<xml_test_tdesc>.
	(record_xml_tdesc): Update.
	(maintenance_check_xml_descriptions): Update.
	* target-descriptions.h (record_xml_tdesc): Update comment.