Tests: remove @testable import PackageCollectionsSigning

Use the newly extended APIs for testing and use a non-testable import of
the package, avoiding a recompilation.

Author: compnerd

Tests/PackageCollectionsSigningTests/CertificatePolicyTests.swift

@@ -12,7 +12,7 @@
 
 import Basics
 import _Concurrency
-@testable import PackageCollectionsSigning
+import PackageCollectionsSigning
 import _InternalTestSupport
 import SwiftASN1
 import X509

Tests/PackageCollectionsSigningTests/PackageCollectionSigningTests.swift

@@ -14,7 +14,7 @@ import Basics
 import _Concurrency
 import Foundation
 import PackageCollectionsModel
-@testable import PackageCollectionsSigning
+import PackageCollectionsSigning
 import _InternalTestSupport
 import X509
 import XCTest

Tests/PackageCollectionsSigningTests/SignatureTests.swift

@@ -15,7 +15,7 @@ import Basics
 import _Concurrency
 import Crypto
 import Foundation
-@testable import PackageCollectionsSigning
+import PackageCollectionsSigning
 import _InternalTestSupport
 import X509
 import XCTest

Tests/PackageCollectionsSigningTests/Utilities.swift

@@ -13,7 +13,7 @@
 import Basics
 import Dispatch
 import Foundation
-@testable import PackageCollectionsSigning
+import PackageCollectionsSigning
 import X509
 
 // Set `REAL_CERT_USER_ID` and `REAL_CERT_ORG_UNIT` env vars when running ENABLE_REAL_CERT_TEST tests