Fix how we check if a range expression covers one value.

[grynspan]

My previous fix, #806, had the wrong logic to determine if a confirmation's expected count as-a-RangeExpression had exactly one value, so we were printing sub-optimal messages on test failures. Specifically, this:

await confirmation(expectedCount: 1) { // transformed to 1...1 // fail to confirm }

Would report "1...1 time(s)" instead of "1 time".

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test