msan: update shadow constant in strlen_of_shadow test

strlen_of_shadow hardcodes the mem-to-shadow
constant. That constant was changed in D137666;
this CL updates the test accordingly.
(Other tests will be updated separately.)

Differential Revision: https://reviews.llvm.org/D139027

GitOrigin-RevId: f5e2481b42fb4539232b5621adcfc03d2eeb4eb7
1 file changed