fix: add type generic default in test utils

[wschurman]

Why

b8e07f9 added the TID narrowing, but without the default value for TSelectedFields consumers would have to specify keyof TFields manually. All public APIs for entity should have the default, and internal APIs shouldn't need to specify it since they're fully qualified.

How

Add defaults.

Test Plan

yarn tsc

[codecov]

Codecov Report

Merging #104 (c34174a) into master (29c1a43) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   94.95%   94.95%           
=======================================
  Files          66       66           
  Lines        1624     1624           
  Branches      195      195           
=======================================
  Hits         1542     1542           
  Misses         80       80           
  Partials        2        2           

Flag	Coverage Δ
integration	94.95% <ø> (ø)
unittest	94.95% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ty/src/utils/testing/PrivacyPolicyRuleTestUtils.ts	90.90% <ø> (ø)

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