tests: Test that an SDK can be rebuilt without cleaning up (#79)

Since #68, rebuilding a RHEL SDK bundle has failed with the following error unless the bundle is first deleted:
```
Error: The file “lib64” couldn’t be saved in the folder “rhel-ubi9.sdk” because a file with the same name already exists.
```
This is because the condition check around the cleanup logic was inverted when the check was moved into `LinuxRecipe.swift`.

#### Before: 
https://github.com/apple/swift-sdk-generator/blob/5428358a282d53e4d714b90c0348a293cca0038d/Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator%2BEntrypoint.swift#L51-L52

#### After:
https://github.com/apple/swift-sdk-generator/blob/b94744f2b3d89511145072d907a18c9ea6aa5578/Sources/SwiftSDKGenerator/SwiftSDKRecipes/LinuxRecipe.swift#L129-L130

This PR adds a test which tries to build an SDK twice without cleaning up in between.

Author: euanh

Sources/SwiftSDKGenerator/SwiftSDKRecipes/LinuxRecipe.swift

@@ -126,7 +126,7 @@ public struct LinuxRecipe: SwiftSDKRecipe {
     httpClient client: HTTPClient
   ) async throws -> SwiftSDKProduct {
     let sdkDirPath = self.sdkDirPath(paths: generator.pathsConfiguration)
-    if generator.isIncremental {
+    if !generator.isIncremental {
       try await generator.removeRecursively(at: sdkDirPath)
     }
     try await generator.createDirectoryIfNeeded(at: sdkDirPath)

Tests/SwiftSDKGeneratorTests/EndToEndTests.swift

@@ -100,5 +100,41 @@ final class EndToEndTests: XCTestCase {
       }
     }
   }
+
+  func testRepeatedSDKBuilds() async throws {
+    let fm = FileManager.default
+
+    var packageDirectory = FilePath(#file)
+    packageDirectory.removeLastComponent()
+    packageDirectory.removeLastComponent()
+
+    // Test that an existing SDK can be rebuilt without cleaning up.
+    // Test with no arguments by default:
+    var possibleArguments = [""]
+    do {
+      try await Shell.run("docker ps")
+      possibleArguments.append("--with-docker --linux-distribution-name rhel --linux-distribution-version ubi9")
+    } catch {
+      self.logger.warning("Docker CLI does not seem to be working, skipping tests that involve Docker.")
+    }
+
+    for runArguments in possibleArguments {
+      let testPackageURL = FileManager.default.temporaryDirectory.appendingPathComponent("swift-sdk-generator-test")
+      let testPackageDir = FilePath(testPackageURL.path)
+      try? fm.removeItem(atPath: testPackageDir.string)
+      try fm.createDirectory(atPath: testPackageDir.string, withIntermediateDirectories: true)
+      defer { try? fm.removeItem(atPath: testPackageDir.string) }
+
+      let firstGeneratorOutput = try await Shell.readStdout(
+        "cd \(packageDirectory) && swift run swift-sdk-generator \(runArguments)"
+      )
+      XCTAssert(firstGeneratorOutput.contains("swift experimental-sdk install"))
+
+      let repeatGeneratorOutput = try await Shell.readStdout(
+        "cd \(packageDirectory) && swift run swift-sdk-generator \(runArguments)"
+      )
+      XCTAssert(repeatGeneratorOutput.contains("swift experimental-sdk install"))
+    }
+  }
   #endif
 }