Several problems with test-network Makefile target

In #18240 (moved), cypherpunks pointed out that we should use the same $(EXEEXT) syntax for finding the appropriate tor-gencert and tor-cov-gencert in the test-network target.

There's a couple other problems with that target:

• NETWORK_FLAVOUR=${NETWORK_FLAVOUR:-"bridges+hs"}: the "bridges+hs" network template doesn't exist anymore.
• If $use_coverage_binary is set when src/test/test-network.sh is called, then chutney will still try to use tor-gencert instead of tor-cov-gencert. (Also the filename extension problem mentioned above applies here also.