test: Tidy up test building with SPL

We can in principle add tests to any SPL build, e.g. TPL or VPL. Update
the build rules to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index 6ad82ce..30672a6 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -109,7 +109,7 @@
 libs-y += fs/
 libs-$(CONFIG_SPL_POST_MEM_SUPPORT) += post/drivers/
 libs-$(CONFIG_SPL_NET) += net/
-libs-$(CONFIG_SPL_UNIT_TEST) += test/
+libs-$(CONFIG_$(SPL_TPL_)UNIT_TEST) += test/
 
 head-y		:= $(addprefix $(obj)/,$(head-y))
 libs-y		:= $(addprefix $(obj)/,$(libs-y))