benchtests: make compare_strings.py accept string as attribute value

Commit ac759b1f added attribute
"overlap" to bench-memmove-walk, whose value is a string. This change
makes compare_strings.py fail since benchout_strings.schema.json
requires the values of attributes to be number.

This patch relaxes such constraint.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>