MXS-2131: Add sanity check test

The sanity check replaces several old regression tests and provides a
quick test for checking mainly the readwritesplit routing behavior. It
also checks some of the connection counts and runs queries that once
caused a crash.

The set of tests that the sanity check obsoletes is:

    bug422
    bug469
    bug448
    bug507
    bug509
    bug634
    bug694
    bug669
    bug711
    mxs127
    mxs47
    mxs682_cyrillic
    mxs957
    mxs1786_statistics
    rwsplit_read_only_trx

maxscale-system-test/mariadb_func.h

@@ -323,10 +323,10 @@ public:
         return get_row(m_conn, q);
     }
 
-    std::string field(std::string q)
+    std::string field(std::string q, int idx = 0)
     {
         Row r = get_row(m_conn, q);
-        return r.empty() ? std::string() : r[0];
+        return r.empty() ? std::string() : r[idx];
     }
 
     const char* error() const