`exportTestSecurityContext()` should consider the `SecurityContextRepository`

This PR introduced the SecurityMockMvcResultHandlers with the exportTestSecurityContext method. It works well when using with @WithSecurityContext and the annotation that inherits it.

However, it does not work when not using any of those annotations. In that scenarios, if the Authentication is null inside the TestSecurityContextHolder.getContext(), we should consider looking into the SecurityContextRepository. Something like this:

private static class ExportTestSecurityContextHandler implements ResultHandler {

	@Override
	public void handle(MvcResult result) {
		SecurityContext securityContext = TestSecurityContextHolder.getContext();
		if (securityContext.getAuthentication() == null) {
			SecurityContextRepository securityContextRepository = WebTestUtils.getSecurityContextRepository(result.getRequest());
			securityContext = securityContextRepository.loadContext(new HttpRequestResponseHolder(result.getRequest(), result.getResponse()));
		}
		SecurityContextHolder.setContext(securityContext);
	}

}

Start by locating SecurityMockMvcResultHandlers and the ExportTestSecurityContextHandler implementation, then inspect the existing tests for exportTestSecurityContext. Verify how WebTestUtils obtains the SecurityContextRepository and add coverage for requests without @WithSecurityContext; done means the handler exports the repository context when the test holder has no authentication.