docs(generics): add test for MemberWidth example (#21923)

This PR fixes failing spectests that started after
#21902. This commit was
supposed to be part of that PR, but didn't get pushed prior to it being
merged due to user error (It was ME!)

The test was missing the command-line execopts and known good output to
compare.

TESTING:

- [x] `make spectests` and test `MemberWidth.chpl`

reviewed by @mppf

doc/rst/language/spec/generics.rst

@@ -815,8 +815,18 @@ instead.
          writeln(r.type:string);
       }
 
-   Running this example with ``--sbig=true`` will print out ``R(64)``, and with
-   ``--sbig=false`` or no argument it will print out ``R(32)``.
+
+   .. BLOCK-test-chapelexecopts
+      -sbig=true
+
+
+   .. BLOCK-test-chapeloutput
+      R(64)
+
+
+
+   Running this example with ``-sbig=true`` will print out ``R(64)``, and with
+   ``-sbig=false`` or no argument it will print out ``R(32)``.
 
 .. _Fields_without_Types: