tests: use raw string in test-impexp-branch.t
On first glance, the escaping of \s seems correct. However,
the shell eats the escape and we're left with '\s` in the
written file. Let's use a raw string so we don't have to
double escape.
Differential Revision: https://phab.mercurial-scm.org/D5973
BasedOnStyle: LLVM
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IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2