pickfirst: Ensure pickfirst_test.go runs against both new and old policies

[arjan-bal]

The tests in pickfirst_test.go are meant to test common behaviours of both the new and old pickfirst policies depending on which LB policy is registered under the pick_first name. The tests were only using the old policy due to the usage of the pickfirstBuilder{}. This change uses the balancer registry to get the builder.

This change also removed the Leaf part in the test names, since these tests are not only for the the new pickfirst.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.89%. Comparing base (36d5ca0) to head (b7e0a51).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7859      +/-   ##
==========================================
+ Coverage   81.69%   81.89%   +0.19%     
==========================================
  Files         374      375       +1     
  Lines       37978    37979       +1     
==========================================
+ Hits        31028    31104      +76     
+ Misses       5638     5577      -61     
+ Partials     1312     1298      -14     

see 25 files with indirect coverage changes

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