Improve Makefile readability

This is supposed to make the Makefile somewhat easier to understand --
but NOT intended to change its behavior/result.

Separate the Emacs and Racket targets.

Replace the `test-x-rkt-files` intermediate variable with two distinct
targets `test-racket-submod` and `test-racket-plain`.

Use more explicit form of `raco test -x`.

Makefile

@@ -1,4 +1,4 @@
-.PHONY : help show-versions clean compile deps minimal-racket-deps test test-elisp test-racket test-slow
+.PHONY : help show-versions clean compile deps minimal-racket-deps test test-elisp test-racket test-racket-submod test-racket-plain test-racket-slow
 
 help:
 	@echo "Targets: show-versions, clean, compile, deps, test, test-elisp, test-racket, test-slow"
@@ -14,6 +14,11 @@ show-versions:
 	@echo `which $(EMACS)`
 	@$(EMACS) --version
 
+test: test-racket test-elisp
+
+######################################################################
+# Emacs
+
 batch-emacs := \
   $(EMACS) --batch -Q -L . \
   --eval '(require (quote package))' \
@@ -55,31 +60,41 @@ deps:
       --eval '(package-install (quote faceup))' \
       --eval '(package-install (quote paredit))'
 
-minimal-racket-deps:
-	$(RACKET) -l raco pkg install --auto \
-     data-lib errortrace-lib macro-debugger-text-lib rackunit-lib \
-     racket-index scribble-lib drracket-tool-text-lib
-
-test: test-racket test-elisp
-
 test-elisp:
 	$(batch-emacs) \
       -l ert \
       -l test/racket-tests.el \
       --eval '(setq racket-program "$(RACKET)")' \
       -f ert-run-tests-batch-and-exit
 
-# Files to test using `raco test -x`.
-test-x-rkt-files := $(wildcard ./racket/*.rkt) $(wildcard ./racket/commands/*.rkt)
+######################################################################
+# Racket
+
+minimal-racket-deps:
+	$(RACKET) -l raco pkg install --auto \
+      data-lib errortrace-lib macro-debugger-text-lib rackunit-lib \
+      racket-index scribble-lib drracket-tool-text-lib
+
+test-racket: test-racket-submod test-racket-plain
+
+# Most tests exist in `test` submodules of ordinary source files,
+#
 # Exclude hash-lang.rkt because it will fail to eval on older Rackets;
 # normally we only dynamic-require it. Furthermore its tests are in
 # ./test/racket/hash-lang-test.rkt.
-test-x-rkt-files := $(filter-out ./racket/hash-lang.rkt, $(test-x-rkt-files))
-
-test-racket:
-	$(RACKET) -l raco test -x $(test-x-rkt-files)
+test-racket-submod:
+	$(RACKET) -l raco test --submodule test --no-run-if-absent \
+      $(filter-out ./racket-hash-lang.rkt, \
+                   $(wildcard ./racket/*.rkt) \
+                   $(wildcard ./racket/commands/*.rkt))
+
+# We do have some files in a special directory that consist solely of
+# tests.
+test-racket-plain:
 	$(RACKET) -l raco test ./test/racket/
 
-test-slow:
+# Some very slow tests segregated in `slow-test` submodules so that
+# they're not run by default.
+test-racket-slow:
 	$(RACKET) -l raco test --submodule slow-test ./racket/imports.rkt
 	$(RACKET) -l raco test --submodule slow-test ./racket/commands/check-syntax.rkt