tests: use go clean

The behavoir after `go test -a` is somewhat surprising,
so add `go clean`, which seems to actuall bring everything
up to date.

golang/go#24355 (comment)

Author: rfjakob

Diff for: Makefile

@@ -81,6 +81,7 @@ format: earlyoom
 	clang-format -i *.h *.c
 
 test: earlyoom
-	# Go does not notice when the C code changes, so we have to
-	# use `go test -a`. See https://github.com/golang/go/issues/24355 .
-	cd tests && go test -a -v
+# Go does not notice when the C code changes, so we have to
+# use `go clean` and `go test -a`.
+# See https://github.com/golang/go/issues/24355 .
+	cd tests && go clean -cache -testcache . && go test -a -v