Check that '/proc/self' exists before using it for tests.

Author: spevans

TestFoundation/TestNSData.swift

@@ -926,6 +926,9 @@ extension TestNSData {
 
     func test_contentsOfZeroFile() {
 #if os(Linux)
+        guard FileManager.default.fileExists(atPath: "/proc/self") else {
+            return
+        }
         let contents = NSData(contentsOfFile: "/proc/self/cmdline")
         XCTAssertNotNil(contents)
         if let contents = contents {