fix(test): disable three more string-pointer-arithmetic tests

After test_string_diff.affine was disabled in PR #75, three sibling
tests carry the same obsolete idiom (treating string literals as raw
Int pointers) and fail typechecking the same way:

- test_string_eq.affine: return s1 - s2 (String subtraction)
- test_string_ptr.affine: return s where s: String, fn returns Int
- test_string_multi.affine: return s1 (same shape)

All needed a cast / __addr_of intrinsic that doesn't exist in v0.1.0.
test_string_simple.affine (return 42 after let s = "Hello") is left
enabled because it doesn't return a String.

Re-enable when the cast story lands.

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath