xdsclient: switch more transport tests to e2e style (2/N)

[easwars]

This PR moves more tests in the transport package to live at the xdsclient package and makes them e2e style.

This PR also makes the stream backoff configurable at the xDS client level. This will be used later on as well, as part of the refactor.

#a71-xds-fallback
#xdsclient-refactor

Addresses #6902

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.83%. Comparing base (9afb232) to head (182188e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7693      +/-   ##
==========================================
+ Coverage   81.79%   81.83%   +0.03%     
==========================================
  Files         361      361              
  Lines       27822    27827       +5     
==========================================
+ Hits        22756    22771      +15     
+ Misses       3863     3856       -7     
+ Partials     1203     1200       -3     

Files with missing lines	Coverage Δ
xds/internal/xdsclient/authority.go	89.24% <100.00%> (+0.03%)	⬆️
xds/internal/xdsclient/client_new.go	86.36% <100.00%> (+0.99%)	⬆️
xds/internal/xdsclient/client_refcounted.go	86.11% <100.00%> (ø)
xds/internal/xdsclient/clientimpl.go	90.00% <ø> (ø)
xds/internal/xdsclient/clientimpl_authority.go	84.90% <100.00%> (-3.56%)	⬇️

... and 23 files with indirect coverage changes

[purnesh42H]

lgtm

[zasweq]

LGTM.