Changes all (remaining) unit tests to use subtests. I pretty much followed the same implementation that some were already using, which is the same as the one from Dave Cheney's blog blog, or one of your own repositories.

This PR is very big, so I did not change anything else except putting the original tests into subtests and fix three (likely) writing issues. To make it easy to find them, see:

• line 79 in graph_test.go (changed *directed to *undirected)
• lines 77 and 152 in path_test.go

Some other considerations:

• I replaced all continue statements with t.SkipNow()
• This won't work if you want to run the tests in parallel, unless you rebind the testcase variable inside each for loop. Or alternatively it could use the new experimental environment variable, but that is only avaliable in go 1.21.
• I use gofmt from within Vscode, and some minor formatting differences may show up on the diff. Let me know if you have any formatter preferences

closes #68