chore: upgrade vaadin-testbench to 9.3.8 (#3173)

Co-authored-by: Zhe Sun <zhe@vaadin.com>
Co-authored-by: Luciano Vernaschi <luciano@vaadin.com>

pom.xml

@@ -78,7 +78,7 @@
     <flow.version>24.7-SNAPSHOT</flow.version>
     <slf4j.version>2.0.9</slf4j.version>
     <spring.boot.version>3.4.2</spring.boot.version>
-    <testbench.version>9.3.7</testbench.version>
+    <testbench.version>9.3.8</testbench.version>
     <jakarta.validation.version>3.0.2</jakarta.validation.version>
     <jakarta.annotation.api.version>2.1.1</jakarta.annotation.api.version>
     <jakarta.activation.api.version>2.1.2</jakarta.activation.api.version>