t1092: add tests for 'git check-attr'

Add tests for `git check-attr`, make sure attribute file does get read from index when path is either inside or outside of sparse-checkout definition. Add a test named 'diff --check with pathspec outside sparse definition'. It starts by disabling the trailing whitespace and space-before-tab checks using the core. whitespace configuration option. Then, it specifically re-enables the trailing whitespace check for a file located in a sparse directory by adding a whitespace=trailing-space rule to the .gitattributes file within that directory. Next, create and populate the folder1 directory, and then add the .gitattributes file to the index. Edit the contents of folder1/a, add it to the index, and proceed to "re-sparsify" 'folder1/' with 'git sparse-checkout reapply'. Finally, use 'git diff --check --cached' to compare the 'index vs. HEAD', ensuring the correct application of the attribute rules even when the file's path is outside the sparse-checkout definition. Mark the two tests 'check-attr with pathspec outside sparse definition' and 'diff --check with pathspec outside sparse definition' as 'test_expect_failure' to reflect an existing issue where the attributes inside a sparse directory are ignored. Ensure that the 'check-attr' command fails to read the required attributes to demonstrate this expected failure. Helped-by: Victoria Dye <vdye@github.com> Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>