tests: add a workaround for SPM tests

The path representation in `AbsolutePath` is incorrect and stores a
relative path as an absolute path (paths with leading `/` or `\` are
*drive* relative, not absolute, absolute paths must start with `[A-Z]:`
or `\\[^\]+\[^\]+\`).  This adds a root directory so that the path to
the manifest is not drive relative.

Author: compnerd

Tests/WorkspaceTests/SourceControlPackageContainerTests.swift

@@ -328,7 +328,7 @@ class SourceControlPackageContainerTests: XCTestCase {
     func testPreReleaseVersions() throws {
         let fs = InMemoryFileSystem()
 
-        let repoPath = AbsolutePath.root
+        let repoPath = AbsolutePath.root.appending("SourceCache")
         let filePath = repoPath.appending("Package.swift")
 
         let specifier = RepositorySpecifier(path: repoPath)