The TestExecInTTY test case is sometimes failing like this:

execin_test.go:332: unexpected carriage-return in output "PID USER TIME COMMAND\r\n 1 root 0:00 cat\r\n 7 root 0:00 ps\r\n"

or this:

execin_test.go:332: unexpected carriage-return in output "PID USER TIME COMMAND\r\n 1 root 0:00 cat\n 7 root 0:00 ps\n"

(this is easy to repro with go test -run TestExecInTTY -count 1000).

This is caused by a race between

• an Init() (in this case it is is (*linuxSetnsInit.Init(), but
  (*linuxStandardInit).Init() is no different in this regard),
  which creates a pty pair, sends pty master to runc, and execs
  the container process,

and

• a parent runc process, which receives the pty master fd and calls
  ClearONLCR() on it.

One way of fixing it would be to add a synchronization mechanism
between these two, so Init() won't exec the process until the parent
sets the flag. This seems excessive, though, as we can just move
the ClearONLCR() call to Init(), putting it right after console.NewPty().

Note that bug was only seen in the TestExecInTTY test case, but
from looking at the code it seems like it can also happen during
runc run or runc exec.

While at it:

• simplify/improve the test case;
• make it repeat 300 times for better chances to catch the bug.

Fixes: #2425