text: Title: Link of type test_case not recognized as proper type in generated report
  Body: ### Bug report:
  
  #### Steps to reproduce:
  - run simple test:
  ```
  import allure 
  
  @allure.testcase('TC-1')
  def test_foo():
    assert 0
  ```
  - generate result: `allure generate --clean %alluredir%`
  - check links section of generated report
  
  #### Actual result:
  Test case link is not recognized as test management system link, but as plain link
  
  #### Expected result:
  Test case link is recognized as TMS link with appropriate icon (or similar distinctive behaviour)
  
  #### Additional info and motivation:
  Although it is minor issue - this is probably due to unsync with changes in report generator.
  If label `@allure.link('TC-1', link_type="tms")` is provided instead - link is generated correctly.
  
  I guess it might be an easy fix - update `allure_commons.types.LinkType.TEST_CASE` value to "tms".
  
  #### Versions:
  - allure-pytest == 2.8.6              
  - allure-python-commons == 2.8.6
  - pytest == 5.3.1
  - allure-commandline(2.13.1)