[gdb/testsuite] Fix gdb.threads/fork-plus-threads.exp with readnow

When running test-case gdb.threads/fork-plus-threads.exp with target board
readnow, I run into:
...
[LWP 9362 exited]^M
[New LWP 9365]^M
[New LWP 9363]^M
[New LWP 9364]^M
FAIL: gdb.threads/fork-plus-threads.exp: detach-on-fork=off: \
  inferior 1 exited (timeout)
...

There is code in the test-case to prevent timeouts with readnow:
...
       -re "Thread \[^\r\n\]+ exited" {
           # Avoid timeout with check-read1
           exp_continue
       }
       -re "New Thread \[^\r\n\]+" {
           # Avoid timeout with check-read1
           exp_continue
       }
...
but this doesn't trigger because we get LWP rather than Thread.

Fix this by making these regexps accept LWP as well.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-05-05  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/fork-plus-threads.exp: Handle "New LWP <n>" and
	"LWP <n> exited" messages.