The original sysinfoex_parse() in MMBroadbandModemHuawei does not handle
unquoted <sysmode_name> and <submode_name> fields in ^SYSINFOEX responses,
which are sen on some Huawei modems (e.g. E303). This patch moves the
^SYSINFOEX parsing code to mm-modem-helpers-huawei.c, fixes the regex for
handling unquoted strings in ^SYSINFOEX responses, and adds unit tests.