cmd/evm: make evm statetest accept non-json files

[holiman]

This fixes a regression introduced recently. Without this fix, it's not possible to use statetests without .json suffix.

This is problematic for goevmlab minimizer, which appends the suffix .min during processing.

Without fix

[user@work go-ethereum]$ go run ./cmd/evm/ statetest --trace --trace.format=json ./00155493-mixed-6.json.min.min
null

With fix

[user@work go-ethereum]$ go run ./cmd/evm/ statetest --trace --trace.format=json ./00155493-mixed-6.json.min.min
{"pc":0,"op":127,"gas":"0x19e701","gasCost":"0x3","memSize":0,"stack":[],"depth":1,"refund":0,"opName":"PUSH32"}
{"pc":33,"op":95,"gas":"0x19e6fe","gasCost":"0x2","memSize":0,"stack":["0xc159603ab9942405d1c9c45c46403949f0540852afe082105fc3a18449358973"],"depth":1,"refund":0,"opName":"PUSH0"}