tests/riotboot: add SLOT_RIOT_ELFS:%.elf=%.bin to BUILD_FILES

- When running `BUILD_IN_DOCKER=1 make -C tests/riotboot test`
  new slot binaries (fw + hdr) need to be generated.
  `%.bin: %.elf` is no defined when building in docker, so the
  fw binaries $(SLOT_RIOT_ELFS:%.elf=%.bin) are added to
  BUILD_FILES

tests/riotboot/Makefile

@@ -21,6 +21,11 @@ QUIET ?= 1
 # Thus default to that (instead of epoch set by makefiles/boot/riotboot.inc.mk).
 APP_VER?=0
 
+# To avoid compilation during the test ensure both slot bin files are always
+# generated and linked this ensures that "BUILD_IN_DOCKER=1 make test" and
+# can rely on them being present without having to trigger re-compilation.
+BUILD_FILES += $(SLOT_RIOT_ELFS:%.elf=%.bin)
+
 # The test needs the linked slot binaries without header in order to be able to
 # create final binaries with specific APP_VER values. The CI RasPi test workers
 # don't compile themselves, thus add the required files here so they will be