tests: posix: xsi_realtime: bump min_ram to 96k

The xsi_realtime testsuite now includes tests for fsync() and
fdatasync() which require a ram-based fat filesystem.

Technically speaking, it would probably not be 100% necessary
to use a ramdisk to test these functions, but then the test
would be limited to running on platforms with some file-system
based storage.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>