Start docs on configuration compilation

[natebosch]

Start some docs on the limitations and workarounds for configuration
compilation of the tests.

[natebosch]

Related:

• Add support for -D arguments #1509 - It's confusing that we don't support -D after dart test since some other subcommands of dart allow the flags in that position.
• "No sound null safety" option is ignored when chrome test executed #1510 - It's confusing that we don't support --no-sound-null-safety since this is a compile time only flag, not a runtime behavior flag, so even with dart --no-sound-null-safety test it doesn't impact test behavior.

My hope is that these docs could be what we point to for questions like these about why it doesn't work.

In my draft I avoided being too concrete about which flags are "runtime behavior" vs compile time, because I don't really want to have to maintain a detailed description of VM implementation details here. I don't know if that leaves things too vague to be useful though?

[natebosch]

Another issue with confusion about the behavior:

dart-lang/sdk#45835

In this case the --enable-experiment used to work because we only had the behavior of --use-data-isolate-strategy. After adding support for the frontend server the default behavior changed, and the new default behavior doesn't support experiment args for VM tests at all.