cmake: Fix build issue with kdevelop 5.2.1 on Ubuntu 18.04

Add KF5::ThreadWeaver as a dependency to KDevTest target.

Because kdevelop 5.2.1 uses include headers from the Threadweaver
library in it's public api, but did not add KF5::ThreadWeaver as
a public dependency, clients get an error when compiling.

BUG:424794
FIXED-IN:2.31.90 (KDE releases 20.07.90)