internal/trace: pass GOTRACEBACK=crash to testprogs

The failures in #70310 are hard to decipher. The cases where the lock is
being held either don't really make sense (the STW failures) or the
goroutine that fails is 'running on another thread' and we don't get a
stack trace. In fact, such a goroutine exists even in the STW cases.
Since reproducing this is going to be hard (very few failures over a 2
year span) let's set GOTRACEBACK=crash for these testprogs so next time
it happens we can see why.

For #70310.

Change-Id: I81a780aa82b173d42973f06911cb243f33352be1
Reviewed-on: https://go-review.googlesource.com/c/go/+/680476
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: mknyszek

src/internal/trace/trace_test.go

@@ -600,6 +600,10 @@ func testTraceProg(t *testing.T, progName string, extra func(t *testing.T, trace
 			godebug += "," + extraGODEBUG
 		}
 		cmd.Env = append(cmd.Env, "GODEBUG="+godebug)
+		if _, ok := os.LookupEnv("GOTRACEBACK"); !ok {
+			// Unless overriden, set GOTRACEBACK=crash.
+			cmd.Env = append(cmd.Env, "GOTRACEBACK=crash")
+		}
 
 		// Capture stdout and stderr.
 		//