Small cleanup to test-spawn.lua

This removes a duplicate test and moves some "spawn with empty string as
argument" up to the long list of similar tests (and adds error
checking).

I do not see the point of the assert(#client.get() == 0) and so it was
just dropped.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/test-spawn.lua

@@ -29,6 +29,9 @@ local steps = {
     error_message = spawn(" ")
     assert(string.find(error_message, 'Text was empty'), error_message)
 
+    error_message = spawn("")
+    assert(string.find(error_message, 'Text was empty'), error_message)
+
     error_message = spawn{}
     assert(string.find(error_message, 'There is nothing to execute'), error_message)
 
@@ -90,17 +93,6 @@ local steps = {
       return true
     end
   end,
-  function()
-    -- Test empty command table
-    spawn{}
-    return true
-  end,
-  function()
-    -- Test empty command string
-    spawn("")
-    assert(#client.get() == 0)
-    return true
-  end,
 }
 
 runner.run_steps(steps)