gsm: fix CSCS=? parsing and add testcases

Some devices (at least one Blackberry we know about) don't include
the () around the response.  Handle that and add testcases for it.

src/mm-modem-helpers.h

@@ -18,6 +18,7 @@
 #define MM_MODEM_HELPERS_H
 
 #include "mm-modem-cdma.h"
+#include "mm-charsets.h"
 
 #define MM_SCAN_TAG_STATUS "status"
 #define MM_SCAN_TAG_OPER_LONG "operator-long"
@@ -51,5 +52,8 @@ gboolean mm_cdma_parse_speri_response (const char *reply,
                                        gboolean *out_roaming,
                                        const char **out_desc);
 
+gboolean mm_gsm_parse_cscs_support_response (const char *reply,
+                                             MMModemCharset *out_charsets);
+
 #endif  /* MM_MODEM_HELPERS_H */