tools: Port 'cov' package from SwiftShader

A library for dealing with coverage information, and compressing CTS test information down into something that can be easily downloaded and visualized in a web page

Change-Id: If025e1555665609f7c963bad14c9c91d8324a390
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113642
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

tools/src/fileutils/fileutils_other.go

@@ -22,6 +22,8 @@ import (
 	"os"
 )
 
+const ExeExt = ""
+
 // IsExe returns true if the file at path is an executable
 func IsExe(path string) bool {
 	s, err := os.Stat(path)

tools/src/fileutils/fileutils_windows.go

@@ -15,6 +15,8 @@
 // Package fileutils contains utility functions for files
 package fileutils
 
+const ExeExt = ".exe"
+
 // IsExe returns true if the file at path is an executable
 func IsExe(path string) bool {
 	return true