dev-db/percona-server: Add test depedency on dev-perl/JSON atom; Raise ulimits for...

dev-db/percona-server: Add test depedency on dev-perl/JSON atom; Raise ulimits for test suite

dev-perl/JSON is used by main.myisam_explain_json_non_select_all, main.myisam_explain_json_non_select_none,
main.innodb_explain_js, on_non_select_all and main.innodb_explain_json_non_select_none test.

Ulimits raised to allow to expand test coverage to include parts.partition_* tests. Also, per
default, some tests are expecting at least 4162 max_open_files or will show a warning.

Package-Manager: Portage-2.3.16, Repoman-2.3.6