fix issue with shared pointer being garbage collected in meshblock iterator performance test

[Yurlungur]

PR Summary

Fixes bug accidentally introduced into develop in PR #617. The issue is only in the tests. MeshBlockData::PackVariables now requires a valid MeshBlock to access to set the coords object. A shared pointer to a MeshBlock is initialized in this test, but it's garbage collected away before it can be used. This PR moves it up one scope so it isn't garbage collected.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• CI has been triggered on Darwin for performance regression tests.
• (@lanl.gov employees) Update copyright on changed files

[pgrete]

Looks good. I'll trigger the full suite now, just to be sure.

[codecov]

Codecov Report

Merging #618 (7011c7f) into develop (c99874d) will increase coverage by 0.15%.
The diff coverage is 64.37%.

@@             Coverage Diff             @@
##           develop     #618      +/-   ##
===========================================
+ Coverage    54.10%   54.25%   +0.15%     
===========================================
  Files          124      125       +1     
  Lines        13606    13699      +93     
===========================================
+ Hits          7361     7433      +72     
- Misses        6245     6266      +21     

Impacted Files	Coverage Δ
.../stochastic_subgrid/stochastic_subgrid_package.cpp	0.00% <0.00%> (ø)
src/defs.hpp	100.00% <ø> (ø)
src/kokkos_abstraction.hpp	95.91% <ø> (ø)
src/mesh/meshblock.hpp	100.00% <ø> (ø)
src/outputs/formatted_table.cpp	0.00% <0.00%> (ø)
src/outputs/outputs.hpp	64.28% <ø> (ø)
src/outputs/vtk.cpp	0.00% <0.00%> (ø)
src/utils/utils.hpp	24.00% <24.00%> (ø)
src/parthenon_manager.cpp	81.45% <33.33%> (+2.54%)	⬆️
src/interface/update.cpp	25.71% <50.00%> (ø)
... and 18 more

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[Yurlungur]

Looks like all tests pass. @brryan @lroberts36 @jdolence @forrestglines can someone just quickly add one more approval?

[brryan]

I've run into this kind of problem before too. LGTM