test: initialize platform in NodeTestFixture

Calling TearDown() without Setup() first is not allowed, but
this should fix a coverity warning:

	*** CID 166971:  Uninitialized members  (UNINIT_CTOR)
	/test/cctest/node_test_fixture.h: 97 in NodeTestFixture::NodeTestFixture()()
	91         v8::V8::ShutdownPlatform();
	92         delete platform_;
	93         platform_ = nullptr;
	94       }
	95
	96      private:
	>>>     CID 166971:  Uninitialized members  (UNINIT_CTOR)
	>>>     The compiler-generated constructor for this class does not initialize "platform_".
	97       v8::Platform* platform_;
	98     };

Author: sam-github

test/cctest/node_test_fixture.h

@@ -79,6 +79,8 @@ class NodeTestFixture : public ::testing::Test {
   ArrayBufferAllocator allocator_;
   v8::Isolate* isolate_;
 
+  NodeTestFixture() : isolate_(nullptr), platform_(nullptr) {}
+
   virtual void SetUp() {
     platform_ = v8::platform::CreateDefaultPlatform();
     v8::V8::InitializePlatform(platform_);
@@ -88,6 +90,8 @@ class NodeTestFixture : public ::testing::Test {
   }
 
   virtual void TearDown() {
+    isolate_->Dispose();
+    isolate_ = nullptr;
     v8::V8::ShutdownPlatform();
     delete platform_;
     platform_ = nullptr;