browser: gosh: switch from Logf/Fatalf to TB, and related

This change makes most of the changes described in the final
proposal in https://v.io/i/1128 - namely, it makes all the
requisite API changes.

Things not done in this change:
- "Good PC" logging
- Printing Cmd stdout/stderr on failure

Those will be done in subsequent changes.

MultiPart: 6/9
Change-Id: I87c5c6882ef649348bdd8b4b6454c5d1154f9990
diff --git a/go/src/v.io/x/browser/runner/main.go b/go/src/v.io/x/browser/runner/main.go
index 98ec54e..c23d0b9 100644
--- a/go/src/v.io/x/browser/runner/main.go
+++ b/go/src/v.io/x/browser/runner/main.go
@@ -136,8 +136,9 @@
 	exitOnError(err, "Could not make temp directory")
 
 	vars := map[string]string{}
-	sh := v23test.NewShell(nil, v23test.Opts{ChildOutputDir: "tmp/runner"})
+	sh := v23test.NewShell(nil, nil)
 	defer sh.Cleanup()
+	sh.ChildOutputDir = "tmp/runner"
 	ctx := sh.Ctx
 
 	// Run a mounttable for tests