FunctionalTests: fail fast when shared enlistment mount is dead

When GitRepoTests uses a shared enlistment (enlistmentPerTest=false),
check IsMounted() in SetupForTest before running git commands. If the
mount process crashed during a previous test, all remaining tests in
the fixture would fail with the same unhelpful 'does not appear to be
mounted' error from the pre-command hook. The early check produces a
clear message pointing to the earlier root-cause failure.

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

Author: tyrielv

GVFS/GVFS.FunctionalTests/Tests/GitCommands/GitRepoTests.cs

@@ -137,6 +137,13 @@ public virtual void SetupForTest()
             {
                 this.CreateEnlistment();
             }
+            else if (!this.Enlistment.IsMounted())
+            {
+                Assert.Fail(
+                    "GVFS mount is not running for the shared enlistment. " +
+                    "A previous test likely caused the mount process to crash. " +
+                    "Check earlier test failures for the root cause.");
+            }
 
             if (this.validateWorkingTree == Settings.ValidateWorkingTreeMode.SparseMode)
             {