[6.2][Test] Correctly gate new VarArgs test on new runtime.

The empty-arguments test fails when run against an older runtime, so don't test it there.

I tried to do this before, but got the condition backwards.

rdar://146839898
(cherry picked from commit b2f1b44)

Author: mikeash

test/stdlib/VarArgs.swift

@@ -174,7 +174,7 @@ test_varArgs6()
 
 func test_varArgs7() {
 #if canImport(Darwin) && arch(arm64)
-  let canTest = if #available(SwiftStdlib 6.2, *) { false } else { true }
+  let canTest = if #available(SwiftStdlib 6.2, *) { true } else { false }
 #else
   // va_list is more complicated on other targets so that behavior is not the
   // same, skip the test by doing a fake print of the expected output. Also