Update 624-checker-stringops and 911-get-stack-trace.

After https://r8-review.googlesource.com/29460, D8/R8 canonicalize
const-string if possible.

Two ART tests have some constant strings, which are canonicalized by
that optimization, while outputs still expect constant loading.

For 624-checker-stringops, we can use different constants without
changing test expectations.

For 911-get-stack-trace, every `doTest()` starts with, e.g.,

  System.out.println("######");
  System.out.println("message");
  System.out.println("######");

and one of #'s is canonicalized, hence consistenlty shifted offset in
expected outputs.

Test: art/test/testrunner/testrunner.py -b --host -t 624-checker-stringops
Test: art/test/testrunner/testrunner.py -b --host -t 911-get-stack-trace
Bug: 118235919
Change-Id: I4432fdf3e3d9cda111d0726ce930c935f2c334f0