build: fix test-v8 target

Clean the deps/v8 directory before rebuilding node for the hash seed
test. It is necessary to avoid the script added in [1] to use ignored
files while generating `node-debug-support.cc`.

[1]: #14901

PR-URL: #17269
Fixes: nodejs/node-v8#26
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Makefile

@@ -506,6 +506,7 @@ test-v8: v8
         --no-presubmit \
         --shell-dir=$(PWD)/deps/v8/out/$(V8_ARCH).$(BUILDTYPE_LOWER) \
 	 $(TAP_V8)
+	git clean -fdxq -- deps/v8
 	@echo Testing hash seed
 	$(MAKE) test-hash-seed