fix(store): remove circular dependency for mock import

[brandonroberts]

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What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
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[ ] Other... Please describe:

What is the current behavior?

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What is the new behavior?

Does this PR introduce a breaking change?

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Other information

[ngrxbot]

Preview docs changes for 165793f at https://previews.ngrx.io/pr2540-165793f/