test: add libevm test logger for error promotion

[JonathanOppenheimer]

Closes #111. This adds a test logger for libevm, that treats errors as they should be treated.

• Logs at log.LevelWarn and above trigger t.Errorf(), causing tests to fail
• Critical logs fail immediately: Logs at log.LevelCrit trigger t.Fatalf()
• Info/debug logs pass through: Lower severity logs use t.Logf() for informational output

You can use the logger like this:

  import (                                                                                                                                         
      "github.com/ava-labs/libevm/log"                                                                                                             
      "github.com/ava-labs/strevm/saetest"                                                                                                         
  )                                                                                                                                                
                                                                                                                                                   
  func TestSomething(t *testing.T) {                                                                                                                 
     logger := log.NewLogger(saetest.NewTBHandler(t))                                                                                                 
     
     // Or to set globally                                                                                                                            
     log.SetDefault(log.NewLogger(saetest.NewTBHandler(t)))                                                                                                                                                                                                
  }     

I was thinking about adding tests to show this works, but it seemed silly to test a testing utility.

[ARR4N]

Love it! But I think it belongs in libevm. Please put it in ava-labs/libevm/libevm/ethtest.

[JonathanOppenheimer]

Love it! But I think it belongs in libevm. Please put it in ava-labs/libevm/libevm/ethtest.

Got it! Opened a new PR here: ava-labs/libevm#259