refactor: simplify creation of default engine in tests

[scovich]

What changes are proposed in this pull request?

Unit tests have accumulated myriad ways of creating a new default engine instance. Harmonize and simplify them.

Breaking changes

parse_url_opts was renamed as store_from_url_opts

DefaultEngine::try_new was removed.

DefaultEngine::new signature changed.

How was this change tested?

Test-only change.

[codecov]

Codecov Report

❌ Patch coverage is 89.04110% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.85%. Comparing base (70143ca) to head (da3be18).

Files with missing lines	Patch %	Lines
kernel/src/engine/default/storage.rs	76.19%	3 Missing and 2 partials ⚠️
ffi/src/lib.rs	83.33%	0 Missing and 1 partial ⚠️
test-utils/src/lib.rs	83.33%	0 Missing and 1 partial ⚠️
uc-catalog/src/lib.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1437      +/-   ##
==========================================
- Coverage   84.85%   84.85%   -0.01%     
==========================================
  Files         119      119              
  Lines       30961    30952       -9     
  Branches    30961    30952       -9     
==========================================
- Hits        26271    26263       -8     
+ Misses       3408     3406       -2     
- Partials     1282     1283       +1     

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[DrakeLin]

lgtm

[nicklan]

thanks!