Unit Tests include "Ippl.h" and don't use mocks/stubs

[JoMee]

Issue 1: Unit test TUs include entire project

Description:

Each unit test file currently includes the full project headers (through including "Ippl.h"). This leads to:

• Slow rebuilds: each change to IPPL causes all unit tests to be recompiled.
• Unnecesary coupling: Tests depend on unrelated parts of the project.

Proposal: Replace the inclusion of Ippl.h with a minimal set of headers required for the test

Benefits:

• Faster rebuilds.
• Clearer communication what the unit test is related to.

Issue 2: Unit tests don’t isolate components (no mocks/stubs)

Our current "unit" tests run against the full codebase; If we want to test something that interacts with an ippl::Field, we build against IPPL's actual implementation of a Mesh, FieldLayout, Field, partitioning strategy and (more subtly) IPPLs Comms object. This causes the following issues:

• Failures are more difficult to trace: If someone introduces a bug in ippl::Field, it might only show up in a unit test related to interpolation.
• Unclear contracts: Because components are always tested together, we aren’t forced to be precise about each component’s guarantees and interface. This makes components less modular and makes refactoring riskier.

Proposal:

• Introduce some helper code building mocks/stubs in order to facilitate the writing of unit tests in isolation.
• Keep current unit tests but treat them as integration tests.
• Reimplement them again using mocks/stubs.

Benefits:

• True "unit" tests that validate components in isolation.
• Clearer definition of each component’s guarantees and interfaces.
• Easier/safer refactoring.
• Faster to recompile and execute test-suite.