Remove `ErrorStatementAst.cs`

This was an accident.
Rename a little more.
REFACTOR: Move `ParserTests` to `Pash.ParserIntrinsics.Tests`

So directory & namespace that matches code under test
TEST: Make it easier to debug unit tests.

With this change, I was able to attach to the nunit.exe process to debug
my tests.
TEST: `Set-Location ; Get-Location`