Optimize TAP test dependencies build

[renecannao]

It seems there is a bug here, as MySQL tarball is "downloaded" twice:

proxysql/test/deps/Makefile

Lines 38 to 39 in 06844ed

	cd mysql-connector-c && curl -C - -O -s https://cdn.mysql.com//Downloads/MySQL-5.7/mysql-boost-5.7.44.tar.gz || true
	cd mysql-connector-c && wget -nc -q https://cdn.mysql.com//Downloads/MySQL-5.7/mysql-boost-5.7.44.tar.gz || true

In reality both curl and wget are executed with --continue , so the file is downloaded at most only once.
I guess there is both curl and wget to work with either one or the other.

Yet, I think curl and/or wget should be called only if the file is not present, thus compiling is not depending from having an internet connection if the file is already there.
Maybe an easy solution is to make the tarball itself a Makefile target.