fix: cleans up FileWatcherCertificateProvider in XdsSecurityClientServerTest

Author: AgraVator

xds/src/test/java/io/grpc/xds/XdsSecurityClientServerTest.java

@@ -796,6 +796,13 @@ void resolved() {
 
       @Override
       public void shutdown() {
+        for (EquivalentAddressGroup s : servers) {
+          SslContextProviderSupplier sslContextProviderSupplier = s.getAttributes()
+                  .get(SecurityProtocolNegotiators.ATTR_SSL_CONTEXT_PROVIDER_SUPPLIER);
+          if (sslContextProviderSupplier != null) {
+            sslContextProviderSupplier.close();
+          }
+        }
       }
 
       @Override