Repository: go
Issue: x/playground: faketime headers not stripped from os/exec subprocess output
Labels: NeedsInvestigation

### What did you do?

https://play.golang.org/p/Gi7BzeGhs6Q


### What did you expect to see?

Similar output to `go test -v` when running the test locally:
```
$ go test -v
=== RUN   TestPanicDoChan
=== PAUSE TestPanicDoChan
=== CONT  TestPanicDoChan
    main_test.go:55: /tmp/go-build979564443/b001/example.com.test -test.run=TestPanicDoChan -test.v:
        === RUN   TestPanicDoChan
        fatal error: all goroutines are asleep - deadlock!

        goroutine 1 [chan receive]:
        testing.(*T).Run(0xc000001380, 0x550089, 0xf, 0x5581a8, 0x4831c6)
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1203 +0x2da
        testing.runTests.func1(0xc000001200)
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1475 +0x78
        testing.tRunner(0xc000001200, 0xc000072de0)
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1157 +0xef
        testing.runTests(0xc00000c060, 0x627270, 0x1, 0x1, 0x0, 0x0, 0x0, 0x40d3b0)
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1473 +0x2fe
        testing.(*M).Run(0xc000110000, 0x0)
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1381 +0x1eb
        main.main()
                _testmain.go:43 +0x138

        goroutine 6 [chan receive]:
        example%2ecom.TestPanicDoChan(0xc000001380)
                /tmp/tmp.ghGKUvokgN/example.com/main_test.go:39 +0x61e
        testing.tRunner(0xc000001380, 0x5581a8)
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1157 +0xef
        created by testing.(*T).Run
                /usr/local/google/home/bcmills/sdk/gotip/src/testing/testing.go:1202 +0x2b3

        goroutine 7 [chan receive]:
        example%2ecom.TestPanicDoChan.func1.2(0x4134eb, 0xd0, 0x538fe0, 0x50257ab94168e01)
                /tmp/tmp.ghGKUvokgN/example.com/main_test.go:30 +0x4a
        golang.org/x/sync/singleflight.(*Group).doCall(0xc0000524f0, 0xc00005c2a0, 0x0, 0x0, 0xc00003e798)
                /tmp/tmp.ghGKUvokgN/_gopath/pkg/mod/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208/singleflight/singleflight.go:97 +0x2e
        golang.org/x/sync/singleflight.(*Group).Do(0xc0000524f0, 0x0, 0x0, 0xc00003e798, 0x91ad385ba9787, 0xc00003e7c0, 0x4d388f, 0xc000001380, 0x6bd3b)
                /tmp/tmp.ghGKUvokgN/_gopath/pkg/mod/golang.org/x/sync@v0.0.0-20200625203802-6e8e738ad208/singleflight/singleflight.go:67 +0x1c5
        example%2ecom.TestPanicDoChan.func1(0xc0000524f0, 0xc000022300, 0xc000022360)
                /tmp/tmp.ghGKUvokgN/example.com/main_test.go:28 +0x9a
        created by example%2ecom.TestPanicDoChan
                /tmp/tmp.ghGKUvokgN/example.com/main_test.go:24 +0x5c7
--- PASS: TestPanicDoChan (0.01s)
PASS
ok      example.com     0.019s
```

### What did you see instead?

Mangled output containing [faketime](https://github.com/golang/go/blob/a38a917aee626a9b9d5ce2b93964f586bf759ea0/src/runtime/time_fake.go#L75) headers:
```
0xc000060300PBt���`)
        PBt���`	PBt���`/tmp/sandbox650178755/prog.goPBt���`:PBt���`28PBt���` +PBt���`0x9aPBt���`
        PBt���`created by PBt���`main.TestPanicDoChanPBt���`
        PBt���`	PBt���`/tmp/sandbox650178755/prog.goPBt���`:PBt���`24PBt���` +PBt���`0x5c7PBt���`
--- PASS: TestPanicDoChan (0.00s)
PASS
```