Do not measure perform performance in CI

When executing in Swift CI, run the block to make sure it doesn't hit any assertions because we don't look at performance numbers in CI and CI nodes can have variable performance characteristics if they are not bare-metal.

Anywhere else, run XCTest's `measure` function to measure the performance of the block.

Fixes #939
rdar://145139964

Author: ahoppen

Tests/SwiftFormatPerformanceTests/WhitespaceLinterPerformanceTests.swift

@@ -5,11 +5,19 @@ import XCTest
 @_spi(Testing) import _SwiftFormatTestSupport
 
 final class WhitespaceLinterPerformanceTests: DiagnosingTestCase {
+  /// When executing in Swift CI, run the block to make sure it doesn't hit any assertions because we don't look at
+  /// performance numbers in CI and CI nodes can have variable performance characteristics if they are not bare-metal.
+  ///
+  /// Anywhere else, run XCTest's `measure` function to measure the performance of the block.
+  private func measureIfNotInCI(_ block: () -> Void) {
+    if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] != nil {
+      block()
+    } else {
+      measure { block() }
+    }
+  }
+
   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
@@ -50,7 +58,7 @@ final class WhitespaceLinterPerformanceTests: DiagnosingTestCase {
       count: 20
     )
 
-    measure { performWhitespaceLint(input: input, expected: expected) }
+    measureIfNotInCI { performWhitespaceLint(input: input, expected: expected) }
   }
 
   /// Perform whitespace linting by comparing the input text from the user with the expected