Add some non-regression tests for certain issues

[travkin79]

Test cases for issues #1580, #1735, #1768, and #1491.

Unfortunately, it's not that easy to make regression tests since style and layout details may influence the resulting SVG / PNG images. Thus, these are non-regression tests.

[arnaudroques]

Thanks for those tests.
However, it is not clear to me why build.gradle.kts should be changed.
Could you tell us more about this? Thanks!

[travkin79]

Actually changing build.gradle.kts is not necessary for this PR.
I'm working on some other tests that are reading *.puml files in the test folder and I thought it might be useful to add *.puml files to the test folder for additional tests, that's why added this change. But of course, we can omit modifying build.gradle.kts as long as no test actually reads *.puml files from the test folder.

[travkin79]

Thank you, @arnaudroques.