Restore CommonMark testfiles with trailing tabs and spaces

[Witiko]

In lostenderman#2, @lostenderman added testfiles for the CommonMark 0.30 standard. However, trailing tabs and spaces are not preserved in verbatim environments such as \begin{markdown} ... \end{markdown} with the exception of the ConTeXt MkIV format. Therefore, testfiles with trailing tabs and spaces were either removed or altered before and during #226.

Since #347, testfiles can now contain YAML metadata that specify which TeX formats and templates are supported by the testfile. For example, a testfile that supports the ConTeXt MkIV format and the input template of other formats might have the following header:

if: format == 'context-mkiv' or template == 'input'
---

This allows us to restore CommonMark testfiles with trailing tabs and spaces to the state from lostenderman#2.

A list of CommonMark examples with trailing tabs and spaces is given in #347 (comment). The ConTeXt support for trailing tabs and spaces consists of replacing any two tabs/spaces at the end of a line with two tabs, which, unlike spaces, are guaranteed to be preserved by TeX:

markdown/markdown.dtx

Line 33693 in d4c8e22

line = line:gsub("[ \t][ \t]$", "\t\t")

While this is sufficient for content inside of paragraphs, where hard line breaks will always be correctly recognized in ConTeXt, extra training tabs will be produced in (fenced) code blocks. Therefore, testfiles that only require the preservation of trailing tabs and spaces in paragraphs should contain the following condition: format == 'context-mkiv' or template == 'input'. Testfiles that require the preservation of trailing tabs and spaces in (fenced) code blocks should contain the following condition: template == 'input'.