Fix assertion in testSnapshotShutdownProgressTracker()

Relaxes a log expectation assertion from an exact test
of numShards running snapshots to 1-numShards, since it
is possible for some of the shard snapshot statuses to
already be in stage=PAUSED.

Closes #127690

Author: JeremyDahlgren

server/src/internalClusterTest/java/org/elasticsearch/snapshots/SnapshotShutdownIT.java

@@ -540,12 +540,13 @@ public void testSnapshotShutdownProgressTracker() throws Exception {
         mockLog.awaitAllExpectationsMatched();
         resetMockLog();
 
+        assert 1 <= numShards && numShards <= 10;
         mockLog.addExpectation(
-            new MockLog.SeenEventExpectation(
+            new MockLog.PatternSeenEventExpectation(
                 "SnapshotShutdownProgressTracker running number of snapshots",
                 SnapshotShutdownProgressTracker.class.getCanonicalName(),
                 Level.INFO,
-                "*Number shard snapshots running [" + numShards + "].*"
+                ".+Number shard snapshots running \\[" + (numShards < 10 ? "[1-" + numShards + "]" : "([1-9]|10)") + "].+"
             )
         );