tests: remove useless line wrapping in a code fragment embedded in test script

This is a part of preparation to apply checking with check-code.py on
code fragments embedded in *.t test scripts.

This revision avoids error of check-code.py below.

    Use () to wrap long lines in Python, not \

tests/test-commit-multiple.t

@@ -95,8 +95,7 b' now test that we fixed the bug for all s'
   >                                       for f in repo[rev].files())))
   > 
   > repo = hg.repository(uimod.ui.load(), b'.')
-  > assert len(repo) == 6, \
-  >        "initial: len(repo): %d, expected: 6" % len(repo)
+  > assert len(repo) == 6, "initial: len(repo): %d, expected: 6" % len(repo)
   > 
   > replacebyte(b"bugfix", b"u")
   > time.sleep(2)