Some LoadModel tests use default PhaseOffset

[PhilMiller]

Describe the bug

A default-constructed PhaseOffset doesn't actually specify what values should be used. Default initialization maybe ends up with NEXT_PHASE and subphase 0, which is kinda surprising. This got in the way of understanding some test failures, since it accessed a subphase structure that wasn't present. Don't use the default constructed type.

Expected behavior
A clear and concise description of what you expected to happen.

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Platform (please complete the following information):

• Supercomputer or desktop [e.g., Trinity, Sierra)
• Compiler environment [e.g., clang-3.9, gcc8]
• Libraries [e.g., mpich-2, openmpi)
• Commit ID or release of VT [e.g., vt-1.0.0]

Additional context
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[PhilMiller]

Fixing as part of #1059