Tests: remove @testable import SourceControl

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

Author: compnerd

Tests/SourceControlTests/GitRepositoryProviderTests.swift

@@ -13,7 +13,7 @@ import Foundation
 
 import Basics
 import _InternalTestSupport
-@testable import SourceControl
+import SourceControl
 import Testing
 
 struct GitRepositoryProviderTests {

Tests/SourceControlTests/GitRepositoryTests.swift

@@ -12,7 +12,7 @@
 
 @_spi(ProcessEnvironmentBlockShim)
 import Basics
-@testable import SourceControl
+import SourceControl
 import _InternalTestSupport
 import XCTest
 

Tests/SourceControlTests/RepositoryManagerTests.swift

@@ -14,7 +14,7 @@ import Basics
 import _Concurrency
 import PackageModel
 import _InternalTestSupport
-@testable import SourceControl
+import SourceControl
 import XCTest
 
 final class RepositoryManagerTests: XCTestCase {