ConcurrentModificationException when querying /v3/api-docs/{group} concurrently for different groups. Fixes #1641.

Author: bnasslahsen

springdoc-openapi-common/src/test/java/org/springdoc/api/AbstractOpenApiResourceTest.java

@@ -119,6 +119,7 @@ public void setUp() {
 		ReflectionTestUtils.setField(openAPIService, "serverBaseUrlCustomizers", Optional.empty());
 
 		when(openAPIService.getContext()).thenReturn(context);
+		when(openAPIService.build(any())).thenReturn(openAPI);
 
 		when(openAPIBuilderObjectFactory.getObject()).thenReturn(openAPIService);
 		when(springDocProviders.jsonMapper()).thenReturn(Json.mapper());