tests property system.setProperty

[jonenst]

should use @DynamicPropertySource
static void register(DynamicPropertyRegistry registry) {
registry.add("spring.elasticsearch.xxx", () -> yyy);

instead of System.setProperty

see https://github.com/powsybl/powsybl-case-server/pull/11/files#diff-36e0324746b1b66aaa0dfccba891937eb10b1ca4ff7661d3469afd5ef981f152R46

It's a better API to test things rather than hoping that we are the last do to system.setProperty before it is read