cmake: make test-ci target skip building dependencies

[vszakats]

Make test-ci not depend on the testdeps target.

test-ci is designed to run curl tests in CI. In CI we build all
necessary dependencies explicitly beforehand, and they are always ready
when calling the test-ci step. Thus, it isn't necessary to enforce
them via a dependency rule. Dropping it saves redundant work and delay
in CI jobs.

The testdeps dependency should not normally be a problem. It's
supposed to be a no-op if those targets are already built. In practice
however, it causes a delay and/or an actual rebuild of those
dependencies depending on generator (= build tool) used and other
factors.

As observed in the GHA/windows workflow, the testdeps dependency:

• with Ninja, causes no delay, and no extra work:
  https://github.com/curl/curl/actions/runs/10980099984/job/30485440389#step:25:18

• with GNU Make, caused a re-evaluation of testdeps targets,
  but did not actually rebuild them. This re-evaluation took a
  noticeable time (esp. with non-bundled tests):
  https://github.com/curl/curl/actions/runs/10980099984/job/30485440155#step:14:11 (with bundles)
  https://github.com/curl/curl/actions/runs/10973851013/job/30471690331#step:14:11 (w/o bundles)
  verbose: https://github.com/curl/curl/actions/runs/10980506956/job/30486434629#step:14:13

• with MSBuild, caused a re-evaluation of testdeps targets, and
  triggered a rebuild:
  https://github.com/curl/curl/actions/runs/10980099984/job/30485435968#step:14:19 (with bundles)
  https://github.com/curl/curl/actions/runs/10973851013/job/30471689714#step:14:19 (w/o bundles)
  verbose: https://github.com/curl/curl/actions/runs/10980506956/job/30486436368#step:14:48
  It's suspected that our use of
  -DCMAKE_VS_GLOBALS=TrackFileAccess=false in CI is contributing to
  this. This option is supposed to affect incremental builds only. For
  some reason it affects CI builds too, even though they are not
  incremental, and no sources are changed between build steps.
  Reported-by: Aki Sakurai
  Ref: GHA/windows: do not disable TrackFileAccess #14999

Notice that test-* targets depending on testdeps is NOT sufficient
to build everything to run tests, e.g. it misses to build the curl tool,
which is essential. This is also true for autotools' test-ci target
which misses to build libcurl and the curl tool.

Perhaps it'd be best to drop testdeps as a dependency for all
test-* targets and make it official to require building dependencies
manually. Alternatively these targets could be fixed to rebuild
everything necessary to run tests.