MXS-2515: Add KILL QUERY test case

The test checks that queries are killed properly.

maxscale-system-test/kill_query.cpp

@@ -0,0 +1,50 @@
+/**
+ * Test KILL QUERY functionality
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char* argv[])
+{
+    TestConnections test(argc, argv);
+
+    auto conn = test.maxscales->rwsplit();
+    conn.connect();
+    conn.query("CREATE OR REPLACE TABLE test.t1 (id LONGTEXT)");
+
+    for (int x = 0; x < 10; x++)
+    {
+        conn.query("INSERT INTO test.t1 VALUES (REPEAT('a', 5000000))");
+    }
+
+    for (int i = 0; i < 3; i++)
+    {
+        auto a = test.maxscales->rwsplit();
+        auto b = test.maxscales->rwsplit();
+        test.expect(a.connect() && b.connect(), "Connections should work");
+
+        auto id = a.thread_id();
+
+        test.set_timeout(15);
+        std::thread thr(
+            [&]() {
+                // The ALTER should take longer than 15 seconds to complete so that a KILL is required to
+                // interrupt it.
+                test.expect(!a.query("ALTER TABLE test.t1 FORCE"), "ALTER should fail");
+
+                const char* expected = "Query execution was interrupted";
+                test.expect(strstr(a.error(), expected),
+                            "Alter should fail with '%s' but it failed with '%s'",
+                            expected, a.error());
+            });
+
+        test.expect(b.query("KILL QUERY " + std::to_string(id)), "KILL QUERY failed: %s", b.error());
+        thr.join();
+
+        test.stop_timeout();
+    }
+
+    conn.query("DROP TABLE test.t1");
+
+    return test.global_result;
+}