tests: clean up built binaries after running test-fuzz-targets.t

Most users won't notice a change here because they won't have the
fuzzer infra, but by good fortune my workstation has the required bits
and keeps leaving the fuzzer binaries around.

Differential Revision: https://phab.mercurial-scm.org/D6862

tests/test-fuzz-targets.t

@@ -41,3 +41,6 b' Just run the fuzzers for five seconds ea'
   $ ./bdiff -max_total_time 5
   $ ./mpatch -max_total_time 5
   $ ./xdiff -max_total_time 5
+
+Clean up.
+  $ $MAKE -s clean