gdb/testsuite: prefer gdb_test in gdb.dwarf2/calling-convention

Since ed01945057c "Make gdb_test's question non-optional if specified",
if the question and response parameters are given to gdb_test, the
framework enforces that GDB asks the question.  Before this patch, tests
needed to use gdb_test_multiple to enforce this.

This patch updates the gdb.dwarf2/calling-convention.exp testcase to use
gdb_test to check that GDB asks a question.  This replaces the more
complicated gdb_test_multiple based implementation.

Tested on x86_64-gnu-linux.

Change-Id: I7216e822ca68f2727e0450970097d74c27c432fe