[gdb/testsuite] Fix buffer full errors in gdb.mi/mi-sym-info.exp

[gdb/testsuite] Fix buffer full errors in gdb.mi/mi-sym-info.exp

With debug info packages for system libs installed, I run into buffer full
errors with test-case gdb.mi/mi-sym-info.exp.  Fix these using exp_continue.

This exposes timeouts due to gdb taking a long time before starting to print
output.  Fix these using with_timeout_factor.

Tested on x86_64-linux, with make targets check and check-read1.

gdb/testsuite/ChangeLog:

2020-03-13  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/mi-sym-info.exp: Fix buffer full errors, and timeouts.