Skip flaky performance test on Windows (swiftlang#940)

Let's skip `testWhitespaceLinterPerformance` on Windows until someone with more linter performance insights can craft a real fix for swiftlang#939.

Author: weliveindetail

Tests/SwiftFormatPerformanceTests/WhitespaceLinterPerformanceTests.swift

@@ -5,7 +5,11 @@ import XCTest
 @_spi(Testing) import _SwiftFormatTestSupport
 
 final class WhitespaceLinterPerformanceTests: DiagnosingTestCase {
-  func testWhitespaceLinterPerformance() {
+  func testWhitespaceLinterPerformance() throws {
+    #if os(Windows)
+    // https://github.com/swiftlang/swift-format/issues/939
+    throw XCTSkip("This test is flaky on Windows")
+    #endif
     let input = String(
       repeating: """
         import      SomeModule