[gdb/testsuite] Fix remove-inferiors.exp FAIL with readnow board

We see this failure with the readnow board:
...
FAIL: gdb.multi/remove-inferiors.exp: load binary
...

When running with board readnow, an extra message "Expanding full symbols" is
emitted after the "Reading symbols" message, and the regexp corresponding to
the FAIL only allows the first message.

Fix this by allowing the extra message in the regexp.

gdb/testsuite/ChangeLog:

2019-06-11  Tom de Vries  <tdevries@suse.de>

	PR testsuite/24521
	* gdb.multi/remove-inferiors.exp: Allow "Expanding full symbols"
	message.