Ensure a newline is printed after Planning build (#8050)

If a PackageStructure build needs to occur during a `swift test list
--skip-build` command there is no newline printed to the terminal,
causing the first line of listed tests to appear on the same line as the
progress, i.e:

`[1/1] Planning buildTestPackageTests.example()`

This doesn't occur when you omit `--skip-build` because there is code in
the `LLBuildProgressTracker.swift` that calls
`progressAnimation.complete(success:)` when a build completes. Ideally
I'd like to contain this logic to the LLBuildProgressTracker, however
for a PackageStructure build the relevant buildComplete delegate method
is never called.

Author: plemarquand

Sources/Build/BuildOperation.swift

@@ -770,7 +770,15 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         self.current = (buildSystem, tracker)
 
         // Build the package structure target which will re-generate the llbuild manifest, if necessary.
-        return buildSystem.build(target: "PackageStructure")
+        let buildSuccess = buildSystem.build(target: "PackageStructure")
+
+        // If progress has been printed this will add a newline to separate it from what could be
+        // the output of the command. For instance `swift test --skip-build` may print progress for
+        // the Planning Build stage, followed immediately by a list of tests. Without this finialize()
+        // call the first entry in the list appear on the same line as the Planning Build progress.
+        tracker.finalize(success: true)
+
+        return buildSuccess
     }
 
     /// Create the build system using the given build description.

Sources/Build/LLBuildProgressTracker.swift

@@ -187,6 +187,12 @@ final class LLBuildProgressTracker: LLBuildBuildSystemDelegate, SwiftCompilerOut
         }
     }
 
+    public func finalize(success: Bool) {
+        self.queue.async {
+            self.progressAnimation.complete(success: success)
+        }
+    }
+
     // MARK: llbuildSwift.BuildSystemDelegate
 
     var fs: SPMLLBuild.FileSystem? {