Fix badgerengine tests on Windows

[tie]

This PR ensures that we close engine before cleanup in enginetest. On Windows tests were failing with the following error.

2020/10/23 21:27:17 remove C:\Users\RUNNER~1\AppData\Local\Temp\genji245175255/badger\000000.vlog: The process cannot access the file because it is being used by another process.
  
  github.com/dgraph-io/badger/v2/y.Wrap
  	github.com/dgraph-io/badger/v2@v2.2007.2/y/error.go:71
  github.com/dgraph-io/badger/v2/y.Check
  	github.com/dgraph-io/badger/v2@v2.2007.2/y/error.go:43
  github.com/dgraph-io/badger/v2.(*valueLog).createVlogFile.func1
  	github.com/dgraph-io/badger/v2@v2.2007.2/value.go:1008
  github.com/dgraph-io/badger/v2.(*valueLog).createVlogFile
  	github.com/dgraph-io/badger/v2@v2.2007.2/value.go:1022
  github.com/dgraph-io/badger/v2.(*valueLog).open
  	github.com/dgraph-io/badger/v2@v2.2007.2/value.go:1120
  github.com/dgraph-io/badger/v2.Open
  	github.com/dgraph-io/badger/v2@v2.2007.2/db.go:396
  github.com/genjidb/genji/engine/badgerengine.NewEngine
  	github.com/genjidb/genji/engine/badgerengine/engine.go:24
  github.com/genjidb/genji/engine/badgerengine_test.builder.func1
  	github.com/genjidb/genji/engine/badgerengine_test/engine_test.go:22
  github.com/genjidb/genji/engine/enginetest.TestStoreNextSequence.func3
  	github.com/genjidb/genji@v0.9.0/engine/enginetest/testing.go:824
  testing.tRunner
  	testing/testing.go:1054
  runtime.goexit
  	runtime/asm_amd64.s:1373

[codecov-io]

Codecov Report

Merging #288 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #288   +/-   ##
=======================================
  Coverage   61.65%   61.65%           
=======================================
  Files          69       69           
  Lines        6496     6496           
=======================================
  Hits         4005     4005           
  Misses       1967     1967           
  Partials      524      524           

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[asdine]

Thanks!