Add delay before writing file to disk in tests

Depending on the system, mtime resolution might not be perfectly accurate. Particularly containers appear to have imprecise mtimes.
Wait a short time period before writing the new file to avoid situations like the following:
 - We index a source file and the unit receives a time stamp and wait for indexing to finish
 - We modify the source file but so quickly after the unit has been modified that the updated source file
   receives the same mtime as the unit file
 - We now assume that the we have an up-to-date index for this source file even though we do not.

Waiting 10ms appears to be enough to avoid this situation on the systems we care about.

rdar://147811044

Author: ahoppen

Sources/SKTestSupport/String+writeWithRetry.swift

@@ -20,6 +20,25 @@ extension String {
   /// access to the file but releases it soon after. Retry to save the file if this happens. This matches what a user
   /// would do.
   package func writeWithRetry(to url: URL) async throws {
+    // Depending on the system, mtime resolution might not be perfectly accurate. Particularly containers appear to have
+    // imprecise mtimes.
+    // Wait a short time period before writing the new file to avoid situations like the following:
+    //  - We index a source file and the unit receives a time stamp and wait for indexing to finish
+    //  - We modify the source file but so quickly after the unit has been modified that the updated source file
+    //    receives the same mtime as the unit file
+    //  - We now assume that the we have an up-to-date index for this source file even though we do not.
+    //
+    // Waiting 10ms appears to be enough to avoid this situation on the systems we care about.
+    //
+    // Do determine the mtime accuracy on a system, run the following bash commands and look at the time gaps between
+    // the time stamps
+    // ```
+    // mkdir /tmp/dir
+    // for x in $(seq 1 1000); do touch /tmp/dir/$x; done
+    // for x in /tmp/dir/*; do stat $x; done | grep Modify | sort | uniq
+    // ```
+    try await Task.sleep(for: .milliseconds(10))
+
     #if os(Windows)
     try await repeatUntilExpectedResult(timeout: .seconds(10), sleepInterval: .milliseconds(200)) {
       do {