[red-knot] More comprehensive `is_assignable_to` tests #38504

Job	Run time
cargo fmt	19s
mkdocs	2m 42s
Determine changes	10s
cargo build (release)	0s
pre-commit	3m 6s
python package	1m 15s
cargo clippy	2m 0s
cargo fuzz build	5m 20s
cargo build (msrv)	4m 16s
benchmarks	5m 1s
cargo shear	22s
cargo test (linux)	2m 18s
cargo test (linux, release)	3m 38s
cargo test (wasm)	1m 35s
cargo test (windows)	5m 12s
test scripts	1m 1s
formatter instabilities and black similarity	0s
test ruff-lsp	35s
ecosystem	2m 37s
fuzz parser	0s
	41m 27s

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