[Rule Request] Don't allow @testable imports outside of tests

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

@testable import is meant to be used in unit tests target only. While it's hard to tell whether we're in an unit test target, we could use some heuristics:

• Is this file importing XCTest/Quick/Nimble? or
• Is there a XCTestCase/QuickSpec subclass?

2. Provide several examples of what would and wouldn't trigger violations.

//ok
@testable import Foo
import XCTest

// not ok
@testable import Foo

4. Should the rule be configurable, if so what parameters should be configurable?5.

• Severity
• Test frameworks - used to configure the search for imports (default: [XCTest, Quick, Nimble])
• Test subclass (default: [XCTestCase, QuickSpec])

6. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a rule as opt-in.

Opt-in. By its nature, this rule will have false positives.

Start with the issue's proposed heuristics and the README.md opt-in-rules guidance. Define the rule around @testable imports, XCTest/Quick/Nimble imports, and XCTestCase/QuickSpec subclasses, then verify the provided allowed and violating examples and the requested configuration options.