core: add testcase for txpool

[holiman]

There are cases where transactions remain in pool.all , despite not being either pending or queued. This PR contains a testcase to reproduce such an occurrence.

#go test . -run TestTransactionCapClearsFromAll
--- FAIL: TestTransactionCapClearsFromAll (0.01s)
	tx_pool_test.go:896: Pending 8, Queued: 0, but pool.all = 16, expected 8
FAIL
FAIL	github.com/ethereum/go-ethereum/core	0.172s

Related to #14669

[holiman]

Added the same test to more places:

#go test . -run .
--- FAIL: TestTransactionPendingGlobalLimiting (0.30s)
	tx_pool_test.go:779: Pool.all size 320 != 160 pending + 0 queued
--- FAIL: TestTransactionCapClearsFromAll (0.01s)
	tx_pool_test.go:822: Pool.all size 16 != 8 pending + 0 queued
--- FAIL: TestTransactionPendingMinimumAllowance (0.02s)
	tx_pool_test.go:864: Pool.all size 20 != 10 pending + 0 queued
--- FAIL: TestTransactionPoolRepricing (0.01s)
	tx_pool_test.go:921: queued transactions mismatched: have 2, want 3
FAIL
FAIL	github.com/ethereum/go-ethereum/core	9.391s