[gdb/testsuite] Add cache_verify option for gdb_caching_procs

[gdb/testsuite] Add cache_verify option for gdb_caching_procs

Test-case gdb.base/gdb-caching-proc.exp tests whether procs declared using
gdb_caching_proc give the same results when called more than once.

While this tests consistency of the procs in the context of that test-case, it
doesn't test consistency across the call sites.

Add a local variable cache_verify to proc gdb_do_cache, that can be set to 1
to verify gdb_caching_proc consistency across the call sites.

Likewise, add a local variable cache_verify_proc to set to the name of the
gdb_caching_proc to verify.  This can f.i. be used when changing an existing
proc into a gdb_caching_proc.

Tested on x86_64-linux, with cache_verify set to both 0 and 1.

gdb/testsuite/ChangeLog:

2020-03-16  Tom de Vries  <tdevries@suse.de>

	* lib/cache.exp (gdb_do_cache): Add and handle local variables
	cache_verify and cache_verify_proc.