test/sandbox: separate test tool source

This improves readability and allows gofmt to format the file.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/sandbox/case/main.go

@@ -0,0 +1,9 @@
+package main
+
+import (
+	"os"
+
+	"git.gensokyo.uk/security/fortify/test/sandbox"
+)
+
+func main() { (&sandbox.T{FS: os.DirFS("/")}).MustCheckFile(os.Args[1], "/tmp/sandbox-ok") }