[clang] [analyzer][Z3] Restore the original timeout of 15s (PR #118291)

[Kristóf Umann via cfe-commits]

Szelethus wrote:

> I also have some just-in-case measurements cooking to confirm that we don't observe any nondeterminism with these values. Previous measurements of mine were made with _all_ configs set to 0. Our downstream release goes off with the patch totally reverted.

Sanity check returned normal, no difference in between the report sets.
https://codechecker-demo.eastus.cloudapp.azure.com/Default/reports?run=qtbase_v6.2.0_determinism_check15A_1&run=libwebm_libwebm-1.0.0.27_determinism_check15A_1&run=vim_v8.2.1920_determinism_check15A_1&newcheck=qtbase_v6.2.0_determinism_check15B_1&newcheck=libwebm_libwebm-1.0.0.27_determinism_check15B_1&newcheck=vim_v8.2.1920_determinism_check15B_1

https://github.com/llvm/llvm-project/pull/118291