Replace "PATHEXT" literals with a constant

pkg/edit/highlight_test.go

@@ -80,10 +80,9 @@ func TestMakeHasCommand(t *testing.T) {
 	oldPath := os.Getenv(util.EnvPATH)
 	defer os.Setenv(util.EnvPATH, oldPath)
 	if runtime.GOOS == "windows" {
-		oldPathExt := os.Getenv("PATHEXT")
-		defer os.Setenv("PATHEXT", oldPathExt)
-		// Forces default value
-		os.Setenv("PATHEXT", "")
+		oldPathExt := os.Getenv(util.EnvPATHEXT)
+		defer os.Setenv(util.EnvPATHEXT, oldPathExt)
+		os.Unsetenv(util.EnvPATHEXT) // force default value
 	}
 
 	// Set up a directory in PATH.

pkg/util/util.go

@@ -2,9 +2,13 @@
 package util
 
 // Environment variables with special significance to Elvish.
+//
+// Note that some of these env vars may be significant only in special
+// circumstances; such as when running unit tests.
 const (
-	EnvHOME  = "HOME"
-	EnvPATH  = "PATH"
-	EnvPWD   = "PWD"
-	EnvSHLVL = "SHLVL"
+	EnvHOME    = "HOME"
+	EnvPATH    = "PATH"
+	EnvPATHEXT = "PATHEXT"
+	EnvPWD     = "PWD"
+	EnvSHLVL   = "SHLVL"
 )