tests : re-enable tests [no ci] (#2977)

This commit re-enables the tests in the build process which are
currently commented out.

It is possible to build the tests using `-DWHISPER_BUILD_TESTS=ON` and
then run a single test using:
```console
$ ctest -R test-whisper-cli-tiny.en --test-dir build
Internal ctest changing into directory: /home/danbev/work/ai/whisper-work/build
Test project /home/danbev/work/ai/whisper-work/build
    Start 2: test-whisper-cli-tiny.en
1/1 Test #2: test-whisper-cli-tiny.en .........   Passed    4.44 sec

100% tests passed, 0 tests failed out of 1

Label Time Summary:
en      =   4.44 sec*proc (1 test)
gh      =   4.44 sec*proc (1 test)
tiny    =   4.44 sec*proc (1 test)

Total Test time (real) =   4.44 sec
```

Some of the tests take a long time to run so it might not be a good idea
to enable them in CI, or perhaps we could only run a subset of the tests
in CI.

CMakeLists.txt

@@ -190,8 +190,8 @@ install(FILES "${CMAKE_CURRENT_BINARY_DIR}/whisper.pc"
 #
 
 if (WHISPER_BUILD_TESTS AND NOT CMAKE_JS_VERSION)
-    #include(CTest)
-    #add_subdirectory(tests)
+    include(CTest)
+    add_subdirectory(tests)
 endif ()
 
 if (WHISPER_BUILD_EXAMPLES)