Limit link-to-link test to Linux systems

Because on macOS we don't have a predictable binary to point to that we
know Shescape knows (note that `/bin/sh` is not a symlink but not
sufficient for identifying the shell).

Author: ericcornelissen

test/_constants.js

@@ -9,6 +9,7 @@ export const echoScript = "test/_echo.js";
 
 export const isMacOS = os.platform() === "darwin";
 export const isWindows = os.platform() === "win32";
+export const isLinux = !isMacOS && !isWindows;
 
 /* Illegal arguments */
 export const illegalArguments = [

test/e2e/_common.js

@@ -61,7 +61,7 @@ export function getTestShells() {
     }
   }
 
-  if (!constants.isWindows) {
+  if (constants.isLinux) {
     const doubleLinkedShell = path.resolve(temp, "double-link", "link-to-link");
     systemShells.push(doubleLinkedShell);
   }