Use IOCapture to silence tests.

[maleadt]

The test output of this package is now simply:

     Testing Running tests...

Showing the output of one test run:
--------------------------------------------------------------------------------
[ Info: Running 1 tests in parallel. If this is too many, specify the `--jobs=N` argument to the tests, or set the `JULIA_CPU_THREADS` environment variable.
               |          | ---------------- CPU ---------------- |
Test  (Worker) | Time (s) |  GC (s) | GC % | Alloc (MB) | RSS (MB) |
basic      (2) |        started at 2025-10-12T08:58:41.233
basic      (2) |     0.03 |   0.00 |  0.0 |       2.13 |   373.42 |

Test Summary: | Pass  Total  Time
  Overall     |    1      1  2.3s
    SUCCESS
--------------------------------------------------------------------------------

Test Summary:                | Pass  Total   Time
ParallelTestRunner           |   21     21  37.8s
  basic test                 |    2      2   8.7s
  custom tests and init code |    3      3   5.1s
  custom worker              |    4      4  10.2s
  failing test               |    6      6   7.4s
  throwing test              |    6      6   6.4s
     Testing ParallelTestRunner tests passed 

[maleadt]

runtests mutates ARGS so this only affected the first invocation.

[giordano]

If a test fails not intentionally do we see the output? Edit: I guess so, this PR handles specially only how tests for this package are run and displayed.