Fix github test run (re: 064baa37); clean up more build sys cruft

The change in .github/workflows/ci.yml (use 'bin/package test' to
run the iffe regression tests as well) caused the GitHub workflow
to fail immediately with a syntax error. This is because iffe.tst
is a ksh93 script, but the runner does not have a ksh93 installed
as a system package.

As of e08ca80d, package prefers a known-good system shell over the
newly compiled shell to stop builds from failing when I break ksh,
making it convenient to fix it. But that change should not apply to
the regression tests; they should use the newly compiled shell.

src/cmd/INIT/package.sh, bin/package:
- Set KEEP_SHELL to 2 when given a SHELL=* argument.
- Before running the regression tests, override the known-good
  shell with the compiled one if $KEEP_SHELL < 2, ensuring that all
  tests, including the iffe ones, are run with the compiled shell.
  This allows 'bin/package test' to run if the known-good shell is
  not a ksh93, while testing that the compiled shell successfully
  runs iffe. (re: e08ca80d)
- Standardise 'test' command use: -a and -o are deprecated.
- Remove some more unused cruft. (re: 6137b99a)

src/cmd/ksh93/Mamfile:
- Do not override SHELL when running shtests; this is now set
  correctly in 'bin/package test'.

src/cmd/INIT/rt.sh:
- Removed. This regression test output filter was only used with
  nmake, which we deleted. (re: 2940b3f5, 6cc2f6a0, aa601a39)

src/cmd/ksh93/Mamfile

@@ -1467,6 +1467,6 @@ make test
 		make tests/shtests
 		done tests/shtests
 		exec - cd "$PACKAGEROOT/src/cmd/ksh93/tests"
-		exec - SHELL=$INSTALLROOT/bin/ksh "$INSTALLROOT/bin/ksh" ./shtests
+		exec - exec "$SHELL" shtests
 	done test.ksh virtual
 done test dontcare virtual