"Migrating a test from XCTest" does not mention parallel-by-default

[MahdiBM]

"Migrating a test from XCTest" does not mention parallel-by-default, which catches users off-guard when migrating from XCTest to swift-testing.

There should also be a sentence or two explaining what to do to go back to the old behavior of non-parallel tests.

I was in a meeting last night and some folks mentioned this issue.
Apparently Fastlane also does not support having the tests run in parallel mode? so they had to figure out the --no-parallel CLI command and use that to run the tests sequentially.

[grynspan]

We do mention it in Parallelization.md, but a nod here wouldn't be a bad idea.

[iamleeg]

Is there anything left to do here now that #960 is merged?