tests: fix Specified key was too long; in a schema

tests: fix Specified key was too long; in a schema

MediaWikiTestCaseSchemaTest.sql comes with two varchar(255) combined
into a PRIMARY KEY.

That caused the testsuite to fail creating that imagelinks table with:
Error: 1071 Specified key was too long; max key length is 767 bytes

That is the limit for innodb and when using UTF8 it allows up to 3 bytes
per characters. Hence the key can be (255+255)*3 = 1530.

One can tune MySQL to bump the limit to ~ 3k with:
    innodb_large_prefix            = 1

Seems easier to just use smaller fields.

Change-Id: Ic4965b9eddc7ad9105c896e678ab9048ce0be8ef