www-servers/nginx: Add test restriction

www-servers/nginx: Add test restriction

Package does not provide a real test suite. Building with FEATURES=test will
currently end with a failure like

  >>> Test phase: www-servers/nginx-1.10.2-r3
   * Test::Harness Jobs=5
  make --jobs 5 --load-average 7.95 test TEST_VERBOSE=0
  make: *** No rule to make target 'test'.  Stop.
   * ERROR: www-servers/nginx-1.10.2-r3::gentoo failed (test phase):
   *   emake failed

Restricting test will prevent an error like this.

Package-Manager: Portage-2.3.3, Repoman-2.3.1