gdb.base/interrupt.exp: Fix race

gdb.base/interrupt.exp: Fix race

Working on splitting gdb and inferior output handling in this test, I
noticed a race that happens to be masked out today.

The test sends "a\n" to the inferior, and then inferior echoes back
"a\n".

If expect manages to read only the first "a\r\n" into its buffer, then
this matches:

    -re "^a\r\n(|a\r\n)$" {

and leaves the second "a\r\n" in output.

Then the next test that processes inferior I/O sends "data\n", and expects:

    -re "^(\r\n|)data\r\n(|data\r\n)$"

which fails given the anchor and given "a\r\n" is still in the buffer.

This is masked today because the test relies on inferior I/O being
done on GDB's terminal, and there are tested GDB commands in between,
which consume the "a\r\n" that was left in the output.

We don't support SunOS4 anymore, so just remove the workaround.

gdb/testsuite/ChangeLog
2015-04-07  Pedro Alves  <palves@redhat.com>

	* gdb.base/interrupt.exp: Don't handle the case of the inferior
	output appearing once only.