Fix broken links to other tests

Originally posted as https://github.com/w3c/csswg-test/issues/1110 by @gsnedders on 16 Aug 2016, 23:19 UTC:

We currently have no general way to solve this because of the build system. My preference here is to resolve this by eliminating the build system. (I guess we could also make them XHTMLonly or whatever the flag is?)

• css21/cascade/specificity-012.xht, line 18, col 6
• css21/cascade/user-stylesheet-013.xht, line 16, col 13
• css21/floats-clear/floats-110.xht, line 20, col 33
• css21/floats-clear/floats-110.xht, line 20, col 68
• css21/margin-padding-clear/margin-collapse-120.xht, line 52, col 16
• css21/margin-padding-clear/margin-collapse-117.xht, line 36, col 49
• css21/margin-padding-clear/margin-collapse-154.xht, line 93, col 49
• css21/normal-flow/inline-block-004.xht, line 24, col 27
• css21/normal-flow/inline-block-005.xht, line 25, col 4
• css21/selectors/first-line-pseudo-011.xht, line 24, col 20
• css21/selectors/link-visited-selector-004.xht, line 19, col 13
• css21/selectors/link-visited-selector-005.xht, line 23, col 13
• css21/selectors/pseudo-009.xht, line 20, col 13

None of these actually break any tests and as such are pretty low priority (they just create needless noise).

Review the listed css21/*.xht files and the build-system handling of links to other tests. Determine how those links should be resolved without creating broken-link noise, then verify the listed locations no longer report broken links.