Merging AbstractClinicServiceTests and ClinicServiceSpringDataJpaTests then using the Spring Boot @DataJpaTest annotation

- remove custom xml view, use springboot builtin xml view
- remove oxm maven dependency
- remove json-simple maven dependency (seems not relevant anymore)
- update vetsXml test using xpath

- In the Jdbc implementation: pets belonging to a visit were not added.
- In the Jpa implementation: query variable was wrong.
- Test case: AbstractClinicServiceTests.shouldFindVisitsByPetId()

RSS is becoming outdated, JSon is a better example

Happens since introduction of new mvc namespace tag for
content-negotiation

There seems to be a conflict between the following:

1) New MVC namespace syntax:
<mvc:content-negotiation use-not-acceptable="true">
	    	<mvc:default-views>
	      		<bean class="org.springframework.web.servlet.view.JstlView">
	      			<property name="url" value="" />
	      		</bean>
	    	</mvc:default-views>
	  	</mvc:content-negotiation>

2) Inside my Unit test:
ResultActions actions =
this.mockMvc.perform(get("/vets.xml").accept(MediaType.TEXT_XML));
        actions.andDo(print()); // action is logged into the console

        actions.andExpect(status().isOk());

Test in error with french default Locale:
org.junit.ComparisonFailure: expected:<[ne peut pas être vide]> but was:<[may not be empty]>
	at org.junit.Assert.assertEquals(Assert.java:115)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at org.springframework.samples.petclinic.model.ValidatorTests.emptyFirstName(ValidatorTests.java:39)

- test didn't have the suffix "Tests" so it was not taken into account
when running Maven in the comand line