[clang] [NFC][analyzer] Split [[assume]] tests to a separate file (PR #130763)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

No problem, in git we can eventually track down and resolve everything.

However, let's try to pay attention to putting logically separate tests into separate files -- our tests are badly disorganized, but we can gradually improve the average quality if we spend some attention on this.

https://github.com/llvm/llvm-project/pull/130763