gdb/testsuite: match complete lines in gdb.base/maint.exp

gdb/testsuite: match complete lines in gdb.base/maint.exp

This thread:

  https://inbox.sourceware.org/gdb-patches/20231003195338.334948-1-thiago.bauermann@linaro.org/

pointed out that within gdb.base/maint.exp, some regexps within a
gdb_test_multiple were failing to match a complete line, while later
regexps within the gdb_test_multiple made use of the '^' anchor, and
so assumed that earlier lines had been completely matched and removed
from expect's buffer.

When testing with READ1 set this assumption was failing.

Fix this by extending the offending patterns with a trailing '\r\n'.

Tested-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>