fix: sets the project for the integration tests (googleapis#386)

The emulator uses the project set in order to list the instance configurations. If the none is set, it gets that information through the gcloud project set, which might mismatch the project being used to create the databases in the integration tests.
This PR hardcodes the project so it matches the instance for the integration tests. Note it can still be overridden, instructions were added in the `CONTRIBUTING.md` file.

Author: thiagotnunes

CONTRIBUTING.md

@@ -48,7 +48,11 @@ integration tests.
 
 ```bash
 export GOOGLE_APPLICATION_CREDENTIALS=/path/to/service/account.json
-mvn -Penable-integration-tests clean verify
+mvn \
+  -Penable-integration-tests \
+  -Dspanner.testenv.instance=projects/<your-project>/instances/<your-instance> \
+  -Dspanner.gce.config.project_id=<your-project> \
+  clean verify
 ```
 
 ## Code Samples

google-cloud-spanner/pom.xml

@@ -69,6 +69,7 @@
           <systemPropertyVariables>
             <spanner.testenv.config.class>com.google.cloud.spanner.GceTestEnvConfig</spanner.testenv.config.class>
             <spanner.testenv.instance>projects/gcloud-devel/instances/spanner-testing</spanner.testenv.instance>
+            <spanner.gce.config.project_id>gcloud-devel</spanner.gce.config.project_id>
           </systemPropertyVariables>
           <forkedProcessTimeoutInSeconds>3000</forkedProcessTimeoutInSeconds>
         </configuration>
@@ -363,4 +364,4 @@
       </build>
     </profile>
   </profiles>
-</project>
+</project>

google-cloud-spanner/src/test/java/com/google/cloud/spanner/IntegrationTestEnv.java

@@ -16,7 +16,6 @@
 
 package com.google.cloud.spanner;
 
-import static com.google.cloud.spanner.testing.EmulatorSpannerHelper.isUsingEmulator;
 import static com.google.common.base.Preconditions.checkState;
 
 import com.google.api.gax.longrunning.OperationFuture;
@@ -80,7 +79,7 @@ protected void before() throws Throwable {
     SpannerOptions options = config.spannerOptions();
     String instanceProperty = System.getProperty(TEST_INSTANCE_PROPERTY, "");
     InstanceId instanceId;
-    if (!instanceProperty.isEmpty() && !isUsingEmulator()) {
+    if (!instanceProperty.isEmpty()) {
       instanceId = InstanceId.of(instanceProperty);
       isOwnedInstance = false;
       logger.log(Level.INFO, "Using existing test instance: {0}", instanceId);