Improve MaxCtrl test coverage

A few missing cases that simple to solve were reported by the coverage
tool.

maxctrl/test/api.js

@@ -16,7 +16,7 @@ describe("API", function() {
             })
     })
 
-    it('sums integer values', function() {
+    it('sums zero integer values', function() {
         return doCommand('api get servers data[].attributes.statistics.connections --sum')
             .then((res) => {
                 js = JSON.parse(res)
@@ -24,6 +24,14 @@ describe("API", function() {
             })
     })
 
+    it('sums non-zero integer values', function() {
+        return doCommand('api get --sum maxscale/threads data[].attributes.stats.reads')
+            .then((res) => {
+                js = JSON.parse(res)
+                js.should.be.a('number')
+            })
+    })
+
     it('does not sum string values', function() {
         return doCommand('api get servers data[].id --sum')
             .then((res) => {

maxctrl/test/call.js

@@ -20,5 +20,10 @@ describe("Module Commands", function() {
             .should.be.rejected
     })
 
+    it('call unknown command', function() {
+        return doCommand('call command mariadbmon log')
+            .should.be.rejected
+    })
+
     after(stopMaxScale)
 });

maxctrl/test/createdestroy.js

@@ -239,5 +239,16 @@ describe("Create/Destroy Commands", function() {
             .should.be.fulfilled
     })
 
+    it('create filter with no parameters', function() {
+        return verifyCommand('create filter test-filter hintfilter',
+                            'filters/test-filter')
+            .should.be.fulfilled
+    })
+
+    it('destroy filter with no parameters', function() {
+        return doCommand('destroy filter test-filter')
+            .should.be.fulfilled
+    })
+
     after(stopMaxScale)
 });

maxctrl/test/diagnostics.js

@@ -10,6 +10,7 @@ var tests = [
     'list modules',
     'list users',
     'list commands',
+    'list threads',
     'show servers',
     'show services',
     'show monitors',
@@ -19,6 +20,7 @@ var tests = [
     'show maxscale',
     'show logging',
     'show threads',
+    'show thread 0',
     'show server server1',
     'show service RW-Split-Router',
     'show monitor MariaDB-Monitor',

maxctrl/test/special.js

@@ -70,6 +70,11 @@ describe("Library invocation", function() {
             .should.be.fulfilled
     })
 
+    it('no command', function() {
+        return ctrl.execute([], {})
+            .should.be.rejected
+    })
+
     it('reject on connection failure', function() {
         stopMaxScale()
             .then(function() {