[Infra] Fix build warning in performance tests (#15063)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

FirebasePerformance/Tests/Unit/Common/FPRDiagnosticsTest.m

@@ -80,7 +80,9 @@ static BOOL classEmitDiagnosticsCalled = NO;
 
 /** Tests that FPRAssert actually asserts (when NS_BLOCK_ASSERTIONS=0|undefined). */
 - (void)testFPRAssert {
-  XCTAssertThrows(FPRAssert(NO, @"This is a failed assert!"));
+  XCTAssertThrows(^{
+    FPRAssert(NO, @"This is a failed assert!");
+  }());
 }
 
 /** Tests emit diagnostics methods. */