gdb.base/commands.exp: Test loop_break and loop_continue in nested loops

gdb.base/commands.exp: Test loop_break and loop_continue in nested loops

This patch improves the loop_break and loop_continue tests to verify
that they work as expected when multiple loops are nested (they affect
the inner loop).

gdb/testsuite/ChangeLog:

	* gdb.base/commands.exp (loop_break_test, loop_continue_test):
	Test with nested loops.