interp: avoid pty.Start in the terminal tests
interp: avoid pty.Start in the terminal tests

See the added comment for the reason.
Before the fix below, the following command would fail within 2s:

    go test -short -race -count=100 -failfast

After the fix below, the command seems to always succeed.

I'll file a bug in creack/pty following this commit.

Fixes #513.

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh