Add a test-install target to the Makefile

This target will be used to check if an installation
of Janet is on the PATH, if native modules can be built
and used via the cook module, etc.

Makefile

@@ -239,6 +239,9 @@ install: $(JANET_TARGET)
 	mkdir -p $(MANPATH)
 	cp janet.1 $(MANPATH)
 
+test-install:
+	cd test/install && janet test
+
 uninstall:
 	-rm $(BINDIR)/../$(JANET_TARGET)
 	-rm -rf $(INCLUDEDIR)