gdb/testsuite: use gdb_test_no_output instead of send_gdb

I noticed gdb.base/ptype-offsets.exp failing occasionally, this was
due to lines like this in the test script:

  send_gdb "set print type hex on\n"

As this does not match the '(gdb)' prompt that is produced, the prompt
would sometimes be matched against the next test, causing the next
test to think its output was missing and fail.

Fix this by switching to:

  gdb_test_no_output "set print type hex on"

gdb/testsuite/ChangeLog:

	* gdb.base/ptype-offsets.exp: Replace use of send_gdb with
	gdb_test_no_output.