select: Fix -none and -clear

If the selection stack only has one element (which it normally does), then
`design->pop_selection()` automatically resets to the default full selection.
This is a problem for `select [-none | -clear]` which were trying to replace the
current selection, but because the pop added an extra element when the `execute`
returned, the extra selection (the one we actually wanted) gets popped too. So
instead, reassign `design->selection()` in the same way as if we called `select
[selection]`.

Also adds selection stack tests, and removes the accidentally-committed
`boxes_dummy.ys`.

tests/select/boxes_dummy.ys

@@ -1,13 +0,0 @@
-read_verilog -specify boxes.v
-clean
-
-echo on
-select -list
-select -count
-select -count *
-select -count top
-select -count top/w:*
-select -list =*
-select -count =*
-select -clear
-select -assert-count 9 =?b

tests/select/boxes_stack.ys

@@ -0,0 +1,29 @@
+read_verilog -specify boxes.v
+clean
+
+# default selection == select *
+select -assert-count 4 *
+select -assert-count 4 %
+
+# -none replaces default selection
+select -none
+select -assert-none %
+select -assert-count 13 =*
+
+# select replaces current selection
+select =*
+select -assert-count 13 %
+
+# -module changes module
+select -module wb
+select -assert-none %
+select -assert-count 5 =*
+
+# -none maintains module
+select -none
+select -assert-count 5 =*
+
+# -clear clears current selection and module
+select -clear
+select -assert-count 4 %
+select -assert-count 13 =*