miner: Ensure test adds transactions synchronously

[piersy]

Description

This fixes a synchronisation error in miner.TestDAFilters transactions were being added to the pool asynchronously and this introduced race conditions that could cause the test to fail randomly, running with a count of 1000 is enough to generate a few failures:

go test ./miner/ -run TestDAFilters -count 1000
--- FAIL: TestDAFilters (0.00s)
    --- FAIL: TestDAFilters/with-tx-filter-max-too-high (0.02s)
        payload_building_test.go:345: Unexpected transaction set: got 1, expected 257
--- FAIL: TestDAFilters (0.00s)
    --- FAIL: TestDAFilters/with-block-filter-max-too-high (0.03s)
        payload_building_test.go:345: Unexpected transaction set: got 1, expected 257
--- FAIL: TestDAFilters (0.00s)
    --- FAIL: TestDAFilters/with-tx-filter-max-too-high (0.02s)
        payload_building_test.go:345: Unexpected transaction set: got 1, expected 257
--- FAIL: TestDAFilters (0.00s)
    --- FAIL: TestDAFilters/with-tx-filter-max-too-high (0.02s)
        payload_building_test.go:345: Unexpected transaction set: got 1, expected 257
--- FAIL: TestDAFilters (0.00s)
    --- FAIL: TestDAFilters/with-nil-tx-filters (0.05s)
        payload_building_test.go:345: Unexpected transaction set: got 1, expected 257
FAIL
FAIL    github.com/ethereum/go-ethereum/miner   28.241s
FAIL