tests: constify command-line arguments

[vszakats]

For libtests, tunits, units.

Also:

• lib3033: tidy up headers.
• lib/netrc: constify an arg in Curl_parsenetrc().

[bagder]

Isn't this a rather pointless exercise for test code? Does it help us in any way?

[vszakats]

It cleans up codepaths a little. Bumped into this when tried defaulting URL to "" for client tests (*while merging clients into libtests)
Then realized via a compiler warning, that I'd need to strip const, which did not smell good.

Can't see any downside to this.

[vszakats]

A one-time potential friction is if there is for new tests pending in PRs.
The entry point signatures of new tests will have to get const added.