Make tests pass on PHP 7.1

[thebrandonallen]

​#WP39277 was fixed in WP 4.7.3, which means PHP 7.1 no longer needs to be allowed to fail, per [11391].

Attached patch moves PHP 7.1 out of allowed failure status, and fixes a type-casting issue in the blogs component causing tests to fail.

[r-a-y]

Nice work, @thebrandonallen.

Should be committed once we start work on 3.0!

[djpaul]

In 11660:

CI: tests must pass on PHP 7.1.

Fixes #7579

Props thebrandonallen

[r-a-y]

In 11668:

Blogs: Ensure offset in bp_blogs_record_existing_blogs() is integer-typecasted.

After r11660, fixes issues with multisite tests failing in PHP 7.1

Props thebrandonallen.

Fixes #7579.