test(autopipeline): consolidate per-topic test files into three

Collapse the ~35 autopipeline_* / osscluster_autopipeline_* test and
benchmark files into three:

  autopipeline_test.go           all blackbox tests (package redis_test),
                                 including the cluster autopipeline tests
  autopipeline_internal_test.go  whitebox tests (package redis): shard-count
                                 decoupling, coalescing, review-fix regressions
  autopipeline_bench_test.go     all benchmarks (package redis_test)

Pure reorganization: no test or benchmark bodies changed, only regrouped.
Counts preserved (63 Test funcs, 16 Benchmarks, 3 Describe suites).
goimports + gofmt applied to the three merged files.

Author: ndyakov