[gdb/testsuite] Make gdb.mi/mi-sym-info.exp more robust against timeouts

Once in a while, I run into this timeout:
...
FAIL: gdb.mi/mi-sym-info.exp: List all variables from debug information \
  only (timeout)
...

I can make the timeout reproducible by setting timeout to 8s (instead of the
default 10s) for the duration of that test.

Make the test-case more stable by fixing all timeouts caused by setting
timeout to 5, either by adding with_timeout_factor, or increasing its factor.

Tested on x86_64-linux.  Also tested in parallel with stress -c 5, to simulate
a busy system in another way.

gdb/testsuite/ChangeLog:

2021-04-30  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/mi-sym-info.exp: Add with_timeout_factor, and increase
	existing timeout factors.