Fix division by zero when sharding is disabled but test filtering is …

…used.

Also allow the test filter to match any part of the test name, not just the whole string. Users can explicitly write `^...$` to force a whole string match.

PiperOrigin-RevId: 607074089

tools/test_discoverer/ObjcTestPrinter.swift

@@ -67,7 +67,7 @@ struct ObjcTestPrinter {
               guard isIncludedByFilter(nameForFiltering(of: test)) else {
                 break
               }
-              if seenTestCount % shardCount == shardIndex {
+              if isIncludedInShard() {
                 shardedSuite.addTest(test)
               }
               seenTestCount += 1

tools/test_discoverer/SymbolGraphTestPrinter.swift

@@ -208,7 +208,7 @@ struct SymbolGraphTestPrinter {
             guard isIncludedByFilter("\\(suiteName)/\\(test.0)") else {
               continue
             }
-            if seenTestCount % shardCount == shardIndex {
+            if isIncludedInShard() {
               shardTests.append(test)
             }
             seenTestCount += 1

tools/test_discoverer/TestPrinterCommon.swift

@@ -75,11 +75,15 @@ func createShardingFilteringTestCollector(extraProperties: String = "") -> Strin
       private func isIncludedByFilter(_ testName: String) -> Bool {
         guard let filter = self.filter else { return true }
         do {
-          return try filter.wholeMatch(in: testName) != nil
+          return try filter.firstMatch(in: testName) != nil
         } catch {
           return false
         }
       }
+
+      private func isIncludedInShard() -> Bool {
+        return shardCount == 0 || seenTestCount % shardCount == shardIndex
+      }
     }
 
     """