Rolling back some earlier Pester Scope changes

Per discussions with @nohwnd, the idea of having `it` be its own Pester scope (clearing `mock`s and `TestDrive` contents the same way `context` and `describe` do) feels a bit awkward in practice.  We're starting to roll back those changes until we find the right balance; for now, `mock`s are still affected by `it` scopes, but `TestDrive` has reverted back to Pester 2.0 behavior (only rolling back `TestDrive` contents when exiting a `context` or `describe` block.)

Also, default behavior of `Assert-MockCalled` has changed to be more in line with Pester 2.0 behavior, possibly allowing old test scripts to function without any modifications.  The default value for the `-Scope` parameter is now either `context` or `describe`, depending on whether there is a currently-active `context` block.  You can still pass in `-Scope <Describe | Context | It>` to override this behavior, if desired.

Tests of these bits have been updated to match the new behavior.  Possibly more changes coming along these lines, once we've tested this revision a bit.