fix: test: TestForkPreMigration hanging when env-var is set

[rjan90]

Related Issues

Fixes #11824

The TestForkPreMigration-test would hang indefinitely if the LOTUS_DISABLE_PRE_MIGRATIONS=1 environment variable was set.

Proposed Changes

Temporarily unsets the LOTUS_DISABLE_PRE_MIGRATIONS environment variable before running the test, and restore it afterward.

Checklist

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