patch 9.1.1536: tests: test_plugin_comment uses wrong :Check command

patch 9.1.1536: tests: test_plugin_comment uses wrong :Check command

Problem:  tests: test_plugin_comment.vim uses :CheckScreenDump check but
          doesn't actually use a screen dump.
Solution: Use :CheckRunVimInTerminal instead, remove a few empty lines.

test_plugin_comment.vim uses :CheckScreenDump to test for the screen
dump feature in each single test case.

However, since we are not actually using any screen dumps, it would be
more correct to use :CheckRunVimInTerminal, since this is the actual
command that we want to run.

And instead of verifying this for each single test, let's just do it
once when sourcing the file. While doing this, also remove some white
spaces.

closes: #17711

Signed-off-by: Christian Brabandt <cb@256bit.org>