It turns out that Spring Boot has property placeholder resolution which allows you to write tests like that:

@SpringBootTest(
  webEnvironment = WebEnvironment.RANDOM_PORT,
  properties = {"rest.clients.echo.base-path=${wiremock.server.baseUrl}"}
)
class EchoServiceTest {
...

And you can even concatenate there other strings like this:

properties = {"rest.clients.echo.base-path=${wiremock.server.baseUrl}/something/else"}