chore: only reset dialect result when it has changed (#1471)

Only change the dialect result that is returned by the mock server
when the dialect has actually changed. This makes the test a bit
quicker, as we also don't have to close the SpannerPool after each
test.

src/test/java/com/google/cloud/spanner/jdbc/SavepointMockServerTest.java

@@ -43,6 +43,7 @@
 import java.sql.SQLException;
 import java.sql.Savepoint;
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Collectors;
 import org.junit.After;
 import org.junit.Before;
@@ -54,6 +55,8 @@
 
 @RunWith(Parameterized.class)
 public class SavepointMockServerTest extends AbstractMockServerTest {
+  private static Dialect currentDialect;
+
   @Parameter public Dialect dialect;
 
   @Parameters(name = "dialect = {0}")
@@ -63,13 +66,18 @@ public static Object[] data() {
 
   @Before
   public void setupDialect() {
-    mockSpanner.putStatementResult(StatementResult.detectDialectResult(this.dialect));
+    // Only reset the dialect result when it has actually changed. This prevents the SpannerPool
+    // from being closed after each test, which again makes the test slower.
+    if (!Objects.equals(this.dialect, currentDialect)) {
+      currentDialect = this.dialect;
+      mockSpanner.putStatementResult(StatementResult.detectDialectResult(this.dialect));
+      SpannerPool.closeSpannerPool();
+    }
   }
 
   @After
   public void clearRequests() {
     mockSpanner.clearRequests();
-    SpannerPool.closeSpannerPool();
   }
 
   private String createUrl() {