fix: Allow derived MockLanguageServer for derived servers.

By changing the visibility of the MockLanguageServer constructor from
private to protected.

org.eclipse.lsp4e.tests.mock/src/org/eclipse/lsp4e/tests/mock/MockLanguageServer.java

@@ -106,7 +106,7 @@ public static void reset(final Supplier<ServerCapabilities> serverConfigurer) {
  INSTANCE = new MockLanguageServer(serverConfigurer);
  }
 
- private MockLanguageServer(final Supplier<ServerCapabilities> serverConfigurer) {
+ protected MockLanguageServer(final Supplier<ServerCapabilities> serverConfigurer) {
  resetInitializeResult(serverConfigurer);
  }