Fix flaky CreatePlaceholderTests stderr comparison

Git's "Updating files" progress output on stderr is
timing-dependent, causing ErrorsShouldMatch to fail when one repo
emits progress and the other does not.

Set GIT_PROGRESS_DELAY=3600 on all test git invocations so
progress output is never emitted for commands completing under an
hour. This eliminates the flake at the source without skipping
error validation.

Assisted-by: Claude Opus 4.6
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>

Author: tyrielv

GVFS/GVFS.FunctionalTests/Tools/GitProcess.cs

@@ -35,6 +35,9 @@ public static ProcessResult InvokeProcess(
             }
 
             processInfo.EnvironmentVariables["GIT_TERMINAL_PROMPT"] = "0";
+            // Suppress progress output (e.g. "Updating files: 100%") which is timing-dependent
+            // and causes flaky stderr comparisons between control and GVFS repos.
+            processInfo.EnvironmentVariables["GIT_PROGRESS_DELAY"] = "3600";
 
             if (environmentVariables != null)
             {