test_gdbprinters: test QMap with a single element

libstdc++'s printers.py contains the following function used by printers
such as StdMapPrinter:
def num_elements(num):
    """Return either "1 element" or "N elements" depending on the argument."""
    return '1 element' if num == 1 else '%d elements' % num

The added check verifies that the regex in QMapPrinter.num_children()
matches the singular form of the word "element".