Port to ecm_add_test to stop skipping all test runs

Summary:
b9aee44d changed `ECM_VERSION` from 1.7.0 to 5.39.0. This caused the
tests to be installed to `build/bin` instead of `build/tests/auto`, but
also made `ctest` and `make test` skip running all tests:
```
$ make test
...
Start  1: imagescalertest
Could not find executable imagescalertest
Looked in the following places:
imagescalertest
imagescalertest
Release/imagescalertest
Release/imagescalertest
Debug/imagescalertest
Debug/imagescalertest
MinSizeRel/imagescalertest
MinSizeRel/imagescalertest
RelWithDebInfo/imagescalertest
RelWithDebInfo/imagescalertest
Deployment/imagescalertest
Deployment/imagescalertest
Development/imagescalertest
Development/imagescalertest
Unable to find executable: imagescalertest
...
```
The CI is also skipping all tests since that commit.

Using `ecm_add_test` instead of manually adding the tests fixes the
issue.

Test Plan:
Remove the complete build dir and run `cmake`. After applying the patch
and rebuilding, `make test` should successfully run all tests again.

Reviewers: #gwenview, muhlenpfordt, ngraham

Reviewed By: #gwenview, muhlenpfordt, ngraham

Subscribers: ngraham, muhlenpfordt, volkov

Differential Revision: https://phabricator.kde.org/D14934