prep-travis-forssh.sh: Check SSH readiness with nc instead of sleeping

[jwodder]

This change causes prep-travis-forssh.sh to wait for the SSH containers to be ready by repeatedly checking whether connections can be made to their SSH ports. This should be more robust than just sleeping for a fixed number of seconds.

[kyleam]

Nice, thank you. I'm going to rebase this to maint.

[codecov]

Codecov Report

Merging #4940 into maint will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            maint    #4940      +/-   ##
==========================================
+ Coverage   89.67%   89.69%   +0.01%     
==========================================
  Files         289      289              
  Lines       40475    40475              
==========================================
+ Hits        36297    36305       +8     
+ Misses       4178     4170       -8     

Impacted Files	Coverage Δ
datalad/support/gitrepo.py	90.45% <0.00%> (+0.07%)	⬆️
datalad/downloaders/base.py	78.92% <0.00%> (+0.35%)	⬆️
datalad/downloaders/http.py	84.94% <0.00%> (+0.38%)	⬆️
datalad/downloaders/tests/test_http.py	91.15% <0.00%> (+1.22%)	⬆️

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