Honor BUILD_TESTING

Summary:
Although ecm_optional_add_subdirectory(autotests) allows to skip
building the autotests with -DBUILD_autotests=FALSE, it doesn't seem
reasonable to have two options controlling similar behaviour and
BUILD_TESTING is the de-facto standard for cmake and already used
when discovering Qt5::Test.

Furthermore it avoids a cmake warning when BUILD_TESTING is set to
FALSE because Qt5::Test is passed to target_link_libraries but
isn't a valid target.

Test Plan: Warning disappears and builds successfully

Reviewers: #kate

Subscribers: kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D14538