Fix Issue 20500 - always pass -unittest and --DRT-testmode=run-main

[wilzbach]

Requires

• Add support for runtimeArgs dlang-tour/core#743
• Add support for runtime arguments + fix CI dlang-tour/core-exec#51

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20500	major	running examples on the home page only displays '1 unittests passed'

[wilzbach]

Both changes are deployed now:

It can also be tested live.
Phobos: http://dtest.dlang.io/artifact/website-bd697944980567f6a4ca6682b542d0cbf1902dba-cc4d855be14f0c9d86e509c0341e91ee/web/phobos-prerelease/std_algorithm_setops.html
dlang.org: http://dtest.dlang.io/artifact/website-bd697944980567f6a4ca6682b542d0cbf1902dba-cc4d855be14f0c9d86e509c0341e91ee/web/index.html
http://dtest.dlang.io/artifact/website-bd697944980567f6a4ca6682b542d0cbf1902dba-cc4d855be14f0c9d86e509c0341e91ee/web/spec/function.html

Though to be fair I don't think -unittest is used often (at all?), so just removing it might have been an option as well. OTOH we now support runtime arguments and this is probably the more intuitive behavior, so we might as well support this.