Exclude test support binder from binder type registry

- Given the TestSupportBinder has its own BinderFactory that returns the TestSupportBinder, we need to exclude the BinderType entry of TestSupportBinder detected from classpath
  - This will make sure to exclude test support binder when TestSupportBinderAutoConfiguration is excluded when using other explicit binder in tests

Resolves #827

Set defaultCandidate to false for TestSupportBinder

 - Set `defaultCandidate` to false for the TestSupportBinder using an environment post processor
 - At BinderFactoryConfiguration, don't override the 'BinderProperties' set via declared binders
   - Set `defaultCandidate` to `true` only for the binders set via BinderTypeRegistry without overriding their binder properties

spring-cloud-stream-test-support/src/main/java/org/springframework/cloud/stream/test/binder/TestBinderEnvironmentPostProcessor.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.stream.test.binder;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.env.EnvironmentPostProcessor;
+import org.springframework.core.env.ConfigurableEnvironment;
+import org.springframework.core.env.MapPropertySource;
+
+/**
+ * An {@link EnvironmentPostProcessor} that sets some configuration properties for {@link TestSupportBinder}.
+ *
+ * @author Ilayaperumal Gopinathan
+ */
+public class TestBinderEnvironmentPostProcessor implements EnvironmentPostProcessor {
+
+	@Override
+	public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
+		Map<String, Object> propertiesToAdd = new HashMap<>();
+		propertiesToAdd.put("spring.cloud.stream.binders.test.defaultCandidate", "false");
+		environment.getPropertySources().addLast(new MapPropertySource("testBinderConfig", propertiesToAdd));
+	}
+}

spring-cloud-stream-test-support/src/main/resources/META-INF/spring.factories

@@ -1,2 +1,4 @@
 org.springframework.boot.autoconfigure.EnableAutoConfiguration:\
-org.springframework.cloud.stream.test.binder.TestSupportBinderAutoConfiguration
+org.springframework.cloud.stream.test.binder.TestSupportBinderAutoConfiguration
+org.springframework.boot.env.EnvironmentPostProcessor=\
+  org.springframework.cloud.stream.test.binder.TestBinderEnvironmentPostProcessor