test: adjust the test use to ls instead of find

Rely on the `ls` behaviour of returning a non-zero if there is no
matching pattern.  This should help make the test pass on Windows
and the other targets.

Author: compnerd

test/Index/index_swift_only_systemmodule.swift

@@ -89,7 +89,7 @@ print(someFunc())
 // RUN:     %s
 //
 // --- Ensure module cache is populated.
-// RUN: find %t/modulecache -maxdepth 1 -name 'SomeModule-*.swiftmodule' | grep .
+// RUN: ls %t/modulecache/SomeModule-*.swiftmodule
 //
 // --- Built with indexing
 // RUN: %target-swift-frontend \