Restore full 'bin/package test' functionality

package now uses mamake to run all available regression tests
(currently iffe, mamake and ksh93) instead of just the ksh tests.

However, as a consequence, passing options or other arguments to
the shtests script via bin/package is no longer possible -- run
bin/shtests directly for that.

src/Mamfile, src/*/Mamfile:
- Make the 'test' virtual target execute subdirectories by
  including the 'install' and 'all' virtual targets within it.

src/cmd/ksh93/Mamfile:
- Simplify and update the script in the 'test' virtual target.

src/cmd/builtin/Mamfile:
- Add dummy 'test' target to avoid an error.

bin/package, src/cmd/INIT/package.sh:
- Run 'mamake test' from the arch/*/src directory. This is the one
  that must be the initial working directory for mamake, though the
  Mamfiles aren't here; mamake finds them via the VPATH variable.
- Update self-doc.

src/cmd/ksh93/Mamfile

@@ -1556,9 +1556,7 @@ make test
 		prev ksh
 		make tests/shtests
 		done tests/shtests
-		exec - silent cmp 2>/dev/null -s ${INSTALLROOT}/bin/ksh ksh 2>/dev/null ||
-		exec - echo "make install to run the tests on the latest ksh" >&2
-		exec - cd
-		exec - SHELL=${INSTALLROOT}/bin/ksh ${INSTALLROOT}/bin/ksh shtests
+		exec - cd "$PACKAGEROOT/src/cmd/ksh93/tests"
+		exec - SHELL=$INSTALLROOT/bin/ksh "$INSTALLROOT/bin/ksh" ./shtests
 	done test.ksh virtual
 done test dontcare virtual