[osh-language] Change the 'unset' builtin to delete the cell.

We don't set the cell to Undef.

Changed assertions in 'spec/assign'.

We no longer match dash/zsh in most cases.

- Case 24 we have unique behavior, but we always did (/)
- We match bash with one case
- We match mksh with two cases

Addresses issue #706.

This is sort of an experiment.  Let's see if ble.sh runs.  Either way
the semantics are simple, and this appears to be closer to what it
wants.

core/state.py

@@ -1428,7 +1428,7 @@ def Unset(self, lval, lookup_mode, strict):
 
     with tagswitch(lval) as case:
       if case(lvalue_e.Named):  # unset x
-        if 1:  # flip this to experiment
+        if 0:  # flip this to experiment
           # This behavior is good for test/spec.sh assign -r 24-27
           name_map[cell_name].val = value.Undef()
           cell.exported = False

spec/assign.test.sh

@@ -301,6 +301,13 @@ echo "x=$x"
 x=temp-binding
 x=mutated-temp
 x=local
+x=mutated-temp
+x=global
+## END
+## OK dash/zsh STDOUT:
+x=temp-binding
+x=mutated-temp
+x=local
 x=
 x=global
 ## END
@@ -343,7 +350,7 @@ x=global
 x=temp-binding f
 echo "x=$x"
 
-## STDOUT:
+## OK dash/zsh STDOUT:
 x=temp-binding
 x=mutated-temp
 x=
@@ -355,7 +362,7 @@ x=mutated-temp
 x=
 x=
 ## END
-## BUG bash STDOUT:
+## STDOUT:
 x=temp-binding
 x=mutated-temp
 x=global
@@ -373,13 +380,13 @@ f() {
 }
 x=global
 f
-## STDOUT:
+## OK dash/bash/zsh STDOUT:
 x=global
 x=local
 - operator = default
 :- operator = default
 ## END
-## BUG mksh STDOUT:
+## STDOUT:
 x=global
 x=local
 - operator = global
@@ -397,13 +404,13 @@ f() {
 }
 x=global
 x=temp-binding f
-## STDOUT:
+## OK dash/zsh STDOUT:
 x=temp-binding
 x=local
 - operator = default
 :- operator = default
 ## END
-## BUG mksh STDOUT:
+## STDOUT:
 x=temp-binding
 x=local
 - operator = temp-binding

test/spec.sh

@@ -416,7 +416,7 @@ builtin-dirs() {
 }
 
 builtin-vars() {
-  sh-spec spec/builtin-vars.test.sh --osh-failures-allowed 2 \
+  sh-spec spec/builtin-vars.test.sh --osh-failures-allowed 1 \
     ${REF_SHELLS[@]} $ZSH $OSH_LIST "$@"
 }