tests/test_util: Fix wrong argument in delta

This spewed errors because the `math` invocation got no second
operand:

    Testing file checks/sigint.fish ... math: Error: Too few arguments
    '1571487730 -'

but only if the `date` didn't do milliseconds, which is the case on
FreeBSD and NetBSD.

(also force the variable to be global - we don't want to have a
universal causing trouble here)

tests/test_util.fish

@@ -137,7 +137,7 @@ end
 # lame timer
 for program in {g,}date
     if command -q $program && $program --version 1>/dev/null 2>/dev/null
-        set milli $program
+        set -g milli $program
         set unit ms
         break
     else
@@ -154,5 +154,5 @@ end
 function delta
     set -q milli[1]
     and math "( "($milli +%s%3N)" - $argv[1])"
-    or math (date +%s) - $starttime
+    or math (date +%s) - $argv[1]
 end