Move get_solver test to more natural location

[bknueven]

Fixes/Resolves: N/A

Summary/Motivation:

In anticipation of moving ipopt-watertap and associated code to its own repository.

Changes proposed in this PR:

• move test for watertap.core.solvers.get_solver into watertap.core.tests.test_solvers

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[lbianchi-lbl]

• This is related to Extract WaterTAP solvers to separately installable package #1511
• We might want to wait until we have a clearer idea of what the final module/import structure should look like before merging this
• OTOH the changes should not have external impact and therefore could be merged in now

[bknueven]

• This is related to Extract WaterTAP solvers to separately installable package #1511

• We might want to wait until we have a clearer idea of what the final module/import structure should look like before merging this

• OTOH the changes should not have external impact and therefore could be merged in now

I went ahead an opened this PR because it seemed like a more natural organization of the existing tests, regardless of the outcome of #1511.

The current organization is a historical accident dating back to when WaterTAP was using the IDAES get_solver function, and probably should have been part of #1353.

[lbianchi-lbl]

Sounds good, thanks for the clarification @bknueven