ci: fix location of fuzzer corpora on VSTS

When using VSTS-based builds, we are in a different location than when
doing Travis builds. Due to this, the relative path to our fuzzer
corpora does not work on VSTS. Fix it by using `${SOURCE_DIR}` instead.

ci/test.sh

@@ -191,7 +191,7 @@ if [ -z "$SKIP_FUZZERS" ]; then
 	echo "##############################################################################"
 
 	for fuzzer in fuzzers/*_fuzzer; do
-		"${fuzzer}" "../fuzzers/corpora/$(basename "${fuzzer%_fuzzer}")" || die $?
+		"${fuzzer}" "${SOURCE_DIR}/fuzzers/corpora/$(basename "${fuzzer%_fuzzer}")" || die $?
 	done
 fi