support/testing/infra/emulator: increase memory size used by Qemu

support/testing/infra/emulator: increase memory size used by Qemu

By default, Qemu emulates a system with 128 MB of RAM. This is not
sufficient for some test cases we have, such as TestPerlDBDmysql,
where the initramfs is quite large. Therefore, this commit extends the
RAM size emulated by Qemu to 256 MB.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/237108668

Thanks to Arnout for the analysis of the issue.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>