revert xds: return all ServerConfig dial options together

[eshitachandwani]

This revert #7680 which :

• Added a single DialOptions() method that returns a slice of all dial options for the type ServerConfig instead of individual methods to get all dial options.
• Changed the test to wait for the custom dialer to be called.

Also reverts #7710 which :

• Added defaultTestTimeout variable which is now unused.

Reverting this is necessary because the wait for dialer is failing in google3.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.82%. Comparing base (5f178a8) to head (4f8aec2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7712      +/-   ##
==========================================
+ Coverage   81.77%   81.82%   +0.04%     
==========================================
  Files         361      361              
  Lines       27826    27828       +2     
==========================================
+ Hits        22756    22769      +13     
+ Misses       3865     3858       -7     
+ Partials     1205     1201       -4     

Files with missing lines	Coverage Δ
internal/xds/bootstrap/bootstrap.go	71.47% <100.00%> (-0.09%)	⬇️
xds/internal/xdsclient/transport/transport.go	93.30% <100.00%> (+0.07%)	⬆️

... and 16 files with indirect coverage changes