There should have been a bit more to the output that you did not include. Which package's tests failed?

It appears to me the error occurred when testing package net, Ian.

This is the full output from my terminal session:
goIssue14377.txt
panic is at line 407.
When I saw the error I opened vi to see the offending source line.
Right after, I inspected my environment variables then I tried to find a script to run tests only. Couldn't find.
Then I tried to build again (by running all.bat) and this time build & test succeeded.

Obs.: it seems test failure at line 940 is unrelated (TestEvalSymlinksCanonicalNames). Issue #13980.

Details about my development box:
OS Name: Microsoft Windows 8 Single Language
OS Version: 6.2.9200 N/A Build 9200
Intel64 Family 6 Model 58 Stepping 9 GenuineIntel ~2400 Mhz

Thanks for the full log.

I think I see an unlikely race condition. In cmd/go, each test sets cmd.Env to the result of envForDir(cmd.Dir,envForDir(cmd.Dir, origEnv). The envForDir function calls mergeEnvLists, which appends to its second argument. Multiple tests are run in parallel. This means that there is a race condition changing the environment used to run the tests. On Windows, the environment is converted into a block of NUL-terminated strings by looping over the environment slice twice. A very precisely timed race could cause the environment slice to change between the two loops, which could cause this error.

I build a version of cmd/go using -race (go build -race -o gorace cmd/go) and then ran gorace test std. I got this, which is the race I am describing.

==================
WARNING: DATA RACE
Read by goroutine 14:
  main.mergeEnvLists()
      /home/iant/go/src/cmd/go/main.go:461 +0x1cc
  main.envForDir()
      /home/iant/go/src/cmd/go/main.go:452 +0x131
  main.(*builder).runTest()
      /home/iant/go/src/cmd/go/test.go:1077 +0x82b
  main.(*builder).do.func1()
      /home/iant/go/src/cmd/go/build.go:1257 +0x56f
  main.(*builder).do.func2()
      /home/iant/go/src/cmd/go/build.go:1314 +0x181

Previous write by goroutine 13:
  main.mergeEnvLists()
      /home/iant/go/src/cmd/go/main.go:463 +0x2aa
  main.envForDir()
      /home/iant/go/src/cmd/go/main.go:452 +0x131
  main.(*builder).runTest()
      /home/iant/go/src/cmd/go/test.go:1077 +0x82b
  main.(*builder).do.func1()
      /home/iant/go/src/cmd/go/build.go:1257 +0x56f
  main.(*builder).do.func2()
      /home/iant/go/src/cmd/go/build.go:1314 +0x181

Goroutine 14 (running) created at:
  main.(*builder).do()
      /home/iant/go/src/cmd/go/build.go:1320 +0x68e
  main.runTest()
      /home/iant/go/src/cmd/go/test.go:595 +0x33f3
  main.main()
      /home/iant/go/src/cmd/go/main.go:181 +0xa38

Goroutine 13 (running) created at:
  main.(*builder).do()
      /home/iant/go/src/cmd/go/build.go:1320 +0x68e
  main.runTest()
      /home/iant/go/src/cmd/go/test.go:595 +0x33f3
  main.main()
      /home/iant/go/src/cmd/go/main.go:181 +0xa38
==================

CL https://golang.org/cl/19513 mentions this issue.