Implement test -d.

Now 'make SHELL=~/git/oil/bin/osh' works again.

core/expr_eval.py

@@ -485,6 +485,9 @@ def Eval(self, node):
        if op_id == Id.BoolUnary_f:
          return stat.S_ISREG(mode)

        if op_id == Id.BoolUnary_d:
          return stat.S_ISDIR(mode)

        raise NotImplementedError(op_id)

      if arg_type == OperandType.Str:

spec/test-builtin.test.sh

@@ -164,4 +164,9 @@ echo status=$?
# OK mksh stdout: status=2
# OK dash stdout: status=1


### -d
test -d $TMP
echo status=$?
test -d $TMP/__nonexistent_Z_Z__
echo status=$?
# stdout-json: "status=0\nstatus=1\n"