test: fix agent lambda tests when run with nyc (elastic#505)

This is a race condition. The tests would call t.end() before all
their assertions had finished. This would not be an issue when running
the tests normally, but when running via nyc it would fail.

test/agent.js

@@ -607,6 +607,8 @@ test('#lambda()', function (t) {
   }
 
   t.test('success - basic callback', function (t) {
+    t.plan(2 + assertTransaction.asserts + assertContext.asserts)
+
     var name = 'greet.hello'
     var input = { name: 'world' }
     var output = 'Hello, world!'
@@ -621,7 +623,6 @@ test('#lambda()', function (t) {
         lambda.invoke(name, input, (err, result) => {
           t.error(err)
           t.equal(result, output)
-          t.end()
         })
       })
       .on('data-transaction', function (data) {
@@ -631,6 +632,8 @@ test('#lambda()', function (t) {
   })
 
   t.test('success - context.succeed', function (t) {
+    t.plan(2 + assertTransaction.asserts + assertContext.asserts)
+
     var name = 'greet.hello'
     var input = { name: 'world' }
     var output = 'Hello, world!'
@@ -645,7 +648,6 @@ test('#lambda()', function (t) {
         lambda.invoke(name, input, (err, result) => {
           t.error(err)
           t.equal(result, output)
-          t.end()
         })
       })
       .on('data-transaction', function (data) {
@@ -655,6 +657,8 @@ test('#lambda()', function (t) {
   })
 
   t.test('success - context.done', function (t) {
+    t.plan(2 + assertTransaction.asserts + assertContext.asserts)
+
     var name = 'greet.hello'
     var input = { name: 'world' }
     var output = 'Hello, world!'
@@ -669,7 +673,6 @@ test('#lambda()', function (t) {
         lambda.invoke(name, input, (err, result) => {
           t.error(err)
           t.equal(result, output)
-          t.end()
         })
       })
       .on('data-transaction', function (data) {