Rename PSM interop fallback test suite to light

[murgatroid99]

This is part of a cross-repository change to generalize the fallback test suite to support other tests, and to change the name for clarity. See also grpc/psm-interop#179.

RELEASE NOTES: n/a

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.20%. Comparing base (6995ef2) to head (6fbbb51).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8350      +/-   ##
==========================================
+ Coverage   82.16%   82.20%   +0.03%     
==========================================
  Files         419      419              
  Lines       42034    42034              
==========================================
+ Hits        34537    34553      +16     
+ Misses       6023     6012      -11     
+ Partials     1474     1469       -5     

see 22 files with indirect coverage changes

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