Fix flaky test `TestPendingTxFilter`

[qdm12]

Run: https://github.com/ava-labs/subnet-evm/actions/runs/12549163673/job/34989721300?pr=1413

Log:

-test.shuffle 1735576483648806000
==================
WARNING: DATA RACE
Read at 0x00c0084afb90 by goroutine [64](https://github.com/ava-labs/subnet-evm/actions/runs/12549163673/job/34989721300?pr=1413#step:8:65):
  runtime.mapaccess2_faststr()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/runtime/map_faststr.go:108 +0x0
  github.com/ava-labs/subnet-evm/eth/filters.TestPendingTxFilterDeadlock()
      D:/a/subnet-evm/subnet-evm/eth/filters/filter_system_test.go:841 +0x569
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:1[68](https://github.com/ava-labs/subnet-evm/actions/runs/12549163673/job/34989721300?pr=1413#step:8:69)9 +0x21e
  testing.(*T).Run.gowrap1()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:1742 +0x44

Previous write at 0x00c0084afb90 by goroutine 109:
  runtime.mapdelete_faststr()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/runtime/map_faststr.go:301 +0x0
  github.com/ava-labs/subnet-evm/eth/filters.(*FilterAPI).NewPendingTransactionFilter.func1()
      D:/a/subnet-evm/subnet-evm/eth/filters/api.go:149 +0x428

Goroutine 64 (running) created at:
  testing.(*T).Run()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:1[74](https://github.com/ava-labs/subnet-evm/actions/runs/12549163673/job/34989721300?pr=1413#step:8:75)2 +0x825
  testing.runTests.func1()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:2161 +0x85
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:1689 +0x21e
  testing.runTests()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:2159 +0x8be
  testing.(*M).Run()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:2027 +0xf17
  main.main()
      _testmain.go:123 +0x2e4

Goroutine 109 (finished) created at:
  github.com/ava-labs/subnet-evm/eth/filters.(*FilterAPI).NewPendingTransactionFilter()
      D:/a/subnet-evm/subnet-evm/eth/filters/api.go:138 +0x406
  github.com/ava-labs/subnet-evm/eth/filters.TestPendingTxFilterDeadlock()
      D:/a/subnet-evm/subnet-evm/eth/filters/filter_system_test.go:[84](https://github.com/ava-labs/subnet-evm/actions/runs/12549163673/job/34989721300?pr=1413#step:8:85)0 +0x51d
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:1689 +0x21e
  testing.(*T).Run.gowrap1()
      C:/hostedtoolcache/windows/go/1.22.10/x64/src/testing/testing.go:1742 +0x44
==================
--- FAIL: TestPendingTxFilterDeadlock (0.60s)
    testing.go:1398: race detected during execution of test
--- FAIL: TestPendingLogsSubscription (1.04s)
    testing.go:1398: race detected during execution of test
--- FAIL: TestLogFilter (1.19s)
    testing.go:1398: race detected during execution of test
--- FAIL: TestPendingTxFilter (1.17s)
    testing.go:1398: race detected during execution of test
FAIL
coverage: 27.5% of statements
FAIL	github.com/ava-labs/subnet-evm/eth/filters	8.0[87](https://github.com/ava-labs/subnet-evm/actions/runs/12549163673/job/34989721300?pr=1413#step:8:88)s