Backslashes become forward slashes in literal types in tests

This works:

uv run mypy -c "from typing import Literal; x: Literal[' \ '] = 1"
<string>:1: error: Incompatible types in assignment (expression has type "Literal[1]", variable has type "Literal[' \\ ']")  [assignment]
Found 1 error in 1 file (checked 1 source file)

However, if you make this test case:

[case testLiteralWithBackslash]
-- This test case is actually more to test our mypy parsing than anything else.
from typing import Literal
x: Literal[" \ "] = "" # E: Incompatible types in assignment (expression has type "Literal['']", variable has type "Literal[' \\ ']")

you get this output:

Expected:
  main:2: error: Incompatible types in assignment (expression has type "Literal['']", variable has type "Literal[' \ ']") (diff)
Actual:
  main:2: error: Incompatible types in assignment (expression has type "Literal['']", variable has type "Literal[' // ']") (diff)

Alignment of first line difference:
  E: ...le has type "Literal[' \ ']")
  A: ...le has type "Literal[' // ']")

This also happens if you expect \\ instead of \. If you specify the variable is of Literal[" \\ "] type, then it becomes four forward slashes. If you do Literal[" \n "] then it becomes /n.

This can create the further odd situation:

[case testLiteralWithBackslash2]
from typing import Literal
x: Literal[" \ "] = " // "

Expected:
Actual:
  main:2: error: Incompatible types in assignment (expression has type "Literal[' // ']", variable has type "Literal[' // ']") (diff)

(if you expect this error verbatim, then at least the test passes!)

Start by reproducing the issue with the reported uv run mypy -c command and the testLiteralWithBackslash and testLiteralWithBackslash2 cases. The report names no source file, so trace how literal-type test cases parse and render expected errors. Done means backslashes and escape sequences remain unchanged in test output and the reproductions pass.