In tests where the logger is a test logger, emitting a trace log in a
background thread while it's shutting down may trigger a panic. Thus
avoid logging Trace if err != nil. Note that we already log an error
when err isn't a trace.
This fixes cases where tests panic with a trace like:
```
panic: Log in goroutine after TestAllocGarbageCollector_MakeRoomFor_MaxAllocs has completed
goroutine 30 [running]:
testing.(*common).logDepth(0xc000aa9e60, 0xc000c4a000, 0xab, 0x3)
/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:680 +0x4d3
testing.(*common).log(...)
/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:662
testing.(*common).Logf(0xc000aa9e60, 0x690b941, 0x4, 0xc001366c00, 0x2, 0x2)
/usr/local/Cellar/go/1.14/libexec/src/testing/testing.go:701 +0x7e
github.com/hashicorp/nomad/helper/testlog.(*writer).Write(0xc000a82a60, 0xc0000b48c0, 0xab, 0x13f, 0x0, 0x0, 0x0)
/Users/notnoop/go/src/github.com/hashicorp/nomad/helper/testlog/testlog.go:34 +0x106
github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog.(*writer).Flush(0xc000a80900, 0xbf9870f000000001, 0x20a87556e, 0x8b12bc0)
/Users/notnoop/go/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog/writer.go:29 +0x14f
github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog.(*intLogger).log(0xc000e2c180, 0xc0003b6880, 0x17, 0x1, 0x6974edc, 0x22, 0xc000db57a0, 0x6, 0x6)
/Users/notnoop/go/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog/intlogger.go:139 +0x15d
github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog.(*intLogger).Trace(0xc000e2c180, 0x6974edc, 0x22, 0xc000db57a0, 0x6, 0x6)
/Users/notnoop/go/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog/intlogger.go:446 +0x7a
github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog.(*interceptLogger).Trace(0xc0002f1ad0, 0x6974edc, 0x22, 0xc000db57a0, 0x6, 0x6)
/Users/notnoop/go/src/github.com/hashicorp/nomad/vendor/github.com/hashicorp/go-hclog/interceptlogger.go:48 +0x9c
github.com/hashicorp/nomad/nomad/drainer.(*drainingJobWatcher).watch(0xc0002f2380)
/Users/notnoop/go/src/github.com/hashicorp/nomad/nomad/drainer/watch_jobs.go:147 +0x1125
created by github.com/hashicorp/nomad/nomad/drainer.NewDrainingJobWatcher
/Users/notnoop/go/src/github.com/hashicorp/nomad/nomad/drainer/watch_jobs.go:89 +0x1e3
FAIL github.com/hashicorp/nomad/client 10.605s
FAIL
```
13381448e0