gdb/testsuite: fix two "maint info line-table"-related tests

Commit 92228a334ba2 ("gdb: small "maintenance info line-table"
readability improvements") change the output format of "maint info
line-table" slightly, adding some empty lines between each
line-table.  This causes two tests to start failing, update them to
account for those empty lines.

Change-Id: I9d33a58fce3e860ba0554b25f5582e8066a5c519