[Test][Driver] Use xargs instead of command substitution

Author: ramonasuncion

test/Driver/filelists.swift

@@ -83,8 +83,8 @@
 // RUN: %swiftc_driver -save-temps -driver-print-jobs %S/../Inputs/empty.swift %t/a.o -lto=llvm-full -target x86_64-apple-macosx10.9 -driver-filelist-threshold=0 -o filelist 2>&1 | tee -a %t/forFilelistCapture | %FileCheck -check-prefix FILELIST %s
 // RUN: sed 's/.*-output-filelist //' %t/forFilelistCapture | sed 's/ .*//' | tail -2 | head -1 > %t/output-filelist
 // RUN: sed 's/.*-filelist //' %t/forFilelistCapture | sed 's/ .*//' | tail -1 | head -1 > %t/input-filelist
-// RUN: cat $(cat %t/output-filelist) | %FileCheck -check-prefix OUTPUT-FILELIST-CONTENTS %s
-// RUN: cat $(cat %t/input-filelist)  | %FileCheck -check-prefix INPUT-FILELIST-CONTENTS %s
+// RUN: cat %t/output-filelist | xargs cat | %FileCheck -check-prefix OUTPUT-FILELIST-CONTENTS %s
+// RUN: cat %t/input-filelist | xargs cat | %FileCheck -check-prefix INPUT-FILELIST-CONTENTS %s
 
 // FILELIST: swift
 // FILELIST-DAG: -emit-bc