test: fix timeDiff to drop old databases before running integration tests (#1874)

timeDiff should be equal to currentTimestamp.getSeconds() - db.getCreateTime().getSeconds(), so as to obtain a non-negative timeDiff value to compare.

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@@ -56,13 +56,13 @@ implementation 'com.google.cloud:google-cloud-spanner'
 If you are using Gradle without BOM, add this to your dependencies
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-spanner:6.23.3'
+implementation 'com.google.cloud:google-cloud-spanner:6.24.0'
 ```
 
 If you are using SBT, add this to your dependencies
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.23.3"
+libraryDependencies += "com.google.cloud" % "google-cloud-spanner" % "6.24.0"
 ```
 
 ## Authentication

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

@@ -180,7 +180,7 @@ private void cleanUpOldDatabases(InstanceId instanceId) {
     while (page != null) {
       for (Database db : page.iterateAll()) {
         try {
-          long timeDiff = db.getCreateTime().getSeconds() - currentTimestamp.getSeconds();
+          long timeDiff = currentTimestamp.getSeconds() - db.getCreateTime().getSeconds();
           // Delete all databases which are more than OLD_DB_THRESHOLD_SECS seconds
           // old.
           if ((db.getId().getDatabase().matches(TEST_DB_REGEX))