gdb/testsuite: fix occasional failure in gdb.mi/mi-multi-commands.exp

gdb/testsuite: fix occasional failure in gdb.mi/mi-multi-commands.exp

In bug PR gdb/29036, another failure was reported for the test
gdb.mi/mi-multi-commands.exp.  This test sends two commands to GDB as
a single write, and then checks that both commands are executed.

The problem that was encountered here is that the output of the first
command, which looks like this:

  ^done,value="\"FIRST COMMAND\""

Is actually produced in parts, first the '^done' is printed, then the
',value="\"FIRST COMMAND\"" is printed.

What was happening is that some characters from the second command
were being echoed after the '^done' had been printed, but before the
value part had been printed.  To avoid this issue I've relaxed the
pattern that checks for the first command a little.  With this fix in
place the occasional failure in this test is no longer showing up.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29036