Un-reverts f2eb5de (swiftlang#2060), which
marked tests runs as started for both XCTest and swift-testing
immediately, but only when not debugging. This version marks tests as
started immediately both when debugging and when running without the
debugger. Original commit message follows.

Previously we were waiting for all parameterized test items to be
emitted on the JSON event stream before starting the `vscode.TestRun`.

This limitation was in place to ensure we had all the test items in test
request sent to VSCode, as we couldn't add test items after a run had
started.

That requirement seems to have been relaxed, or we were just not
refreshing the test explorer correctly after dynamically adding items,
but regardless it looks like we can dynamically add items now.

This means we can correctly mark a test run as "started" immediately
when the user starts compiling a test like we do with XCTest, making the
two testing libraries the same in this regard. It also simplifies the
code, allowing us to accept and inject new parameterized test items at
any time during a run.