test: do not use RESOURCE_EXHAUSTED as it is retryable (#3227)

The test used the error code RESOURCE_EXHAUSTED to indicate that
the server could not create any more sessions. This error code is
now retryable, causing the test to become flaky. Change the error
code to one that is not retryable.

Fixes #3224

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

@@ -207,13 +207,13 @@ public void testSpannerReturnsAllAvailableSessionsAndThenNoSessions()
   }
 
   @Test
-  public void testSpannerReturnsResourceExhausted() throws InterruptedException {
+  public void testSpannerReturnsFailedPrecondition() throws InterruptedException {
     int minSessions = 100;
     int maxSessions = 1000;
     int expectedSessions;
     DatabaseClientImpl client;
     // Make the first BatchCreateSessions return an error.
-    mockSpanner.addException(Status.RESOURCE_EXHAUSTED.asRuntimeException());
+    mockSpanner.addException(Status.FAILED_PRECONDITION.asRuntimeException());
     try (Spanner spanner = createSpanner(minSessions, maxSessions)) {
       // Create a database client which will create a session pool.
       client =

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

@@ -808,7 +808,7 @@ public void batchCreateSessions(
       batchCreateSessionsExecutionTime.simulateExecutionTime(
           exceptions, stickyGlobalExceptions, freezeLock);
       if (sessions.size() >= maxTotalSessions) {
-        throw Status.RESOURCE_EXHAUSTED
+        throw Status.FAILED_PRECONDITION
             .withDescription("Maximum number of sessions reached")
             .asRuntimeException();
       }