test: locate the fuzz sweep in the build tree

fuzz-msg-parse.sh looked for its driver next to the script, which only
holds when srcdir and builddir are the same.  Every VPATH build skipped
the sweep, claiming D-Bus was off:

    SKIP: fuzz-msg-parse.sh

distcheck, and with it the release job, is a VPATH build.  Use
top_builddir, which the test environment already exports.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/fuzz-msg-parse.sh

@@ -10,7 +10,7 @@
 set -eu
 
 TEST_DIR=$(dirname "$0")
-DRIVER="$TEST_DIR/src/fuzz-msg-parse"
+DRIVER="${top_builddir:-$TEST_DIR/..}/test/src/fuzz-msg-parse"
 
 [ -x "$DRIVER" ] || {
     echo "fuzz-msg-parse not built, D-Bus support is off"