gdb/testsuite: use foreach_with_prefix in gdb.threads/non-ldr-exc-*.exp

Replace the manual with_test_prefix in the do_test proc with using
foreach_with_prefix at the top-level.  This helps reduce the indentation
level of the code a bit, and makes the test names in sync with the
variable names used in the code.

gdb/testsuite/ChangeLog:

	* gdb.threads/non-ldr-exc-1.exp: Use foreach_with_prefix.
	(do_test): Don't use with_test_prefix.
	* gdb.threads/non-ldr-exc-2.exp: Use foreach_with_prefix.
	(do_test): Don't use with_test_prefix.
	* gdb.threads/non-ldr-exc-3.exp: Use foreach_with_prefix.
	(do_test): Don't use with_test_prefix.
	* gdb.threads/non-ldr-exc-4.exp: Use foreach_with_prefix.
	(do_test): Don't use with_test_prefix.

Change-Id: I3af1df2eee1a8add427a67b6048bb6dede41cbeb