Fix failing specs

[jjhembd]

Description

This PR fixes two failing specs:

1) renders with distance display condition per instance attribute
     Scene/GroundPolylinePrimitive
     Expected (0, 4) to be undefined.
    at <Jasmine>
    at packages/engine/Specs/Scene/GroundPolylinePrimitiveSpec.js:761:58 <- Build/Specs/SpecList.js:175807:58
    at compare (Specs/addDefaultMatchers.js:314:13 <- Build/Specs/karma-main.js:296:13)
    at <Jasmine>

2) renders a model in fog (sunlight)
     Scene/Model/Model fog
     Expected Uint8Array [ 0, 0, 0, 255 ] not to equal [ 0, 0, 0, 255 ].
    at <Jasmine>
    at packages/engine/Specs/Scene/Model/ModelSpec.js:4680:28 <- Build/Specs/SpecList.js:260288:28
    at compare (Specs/addDefaultMatchers.js:314:13 <- Build/Specs/karma-main.js:296:13)
    at <Jasmine>

The GroundPolylinePrimitive spec is a fragile test that was broken by new terrain heights. See #11807.

The Model spec failure was caused by an unrealistic expectation in the initial test suite from #11744. The assumption was that when the sun is on the opposite side of the earth, the model will appear dark but not exactly black. However, in a spec canvas it does sometimes render as fully black.
The remaining expectations in this spec are sufficient, so I simply deleted the unreliable one.

Testing plan

Verify GroundPolylinePrimitiveSpec and ModelSpec run through locally without errors (CI didn't catch the problems).

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• [ ] I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• [ ] I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[jjspace]

LGTM