tests: avoid echo with backslash escapes

Some shells' echo (e.g. Bash's) do not interpret backslash escapes by
default, while others shells' do (e.g. Dash's). This led to a failure
in test-run-tests.t. We could switch to the more compatible printf,
but using a heredoc seems simpler, so that's what this patch does.

For anyone who wants to test this for themselves, remove the "unset
HGTEST_SHELL" line in helper-runtests.sh and pass
e.g. "--shell=/bin/dash" to the test runner.

Differential Revision: https://phab.mercurial-scm.org/D1699

tests/test-run-tests.t

@@ -703,12 +703,14 b' View the fix'
 
 Accept the fix
 
-  $ echo "  $ echo 'saved backup bundle to \$TESTTMP/foo.hg'" >> test-failure.t
-  $ echo "  saved backup bundle to \$TESTTMP/foo.hg" >> test-failure.t
-  $ echo "  $ echo 'saved backup bundle to \$TESTTMP/foo.hg'" >> test-failure.t
-  $ echo "  saved backup bundle to \$TESTTMP\\foo.hg" >> test-failure.t
-  $ echo "  $ echo 'saved backup bundle to \$TESTTMP/foo.hg'" >> test-failure.t
-  $ echo "  saved backup bundle to \$TESTTMP/*.hg (glob)" >> test-failure.t
+  $ cat >> test-failure.t <<EOF
+  >   $ echo 'saved backup bundle to \$TESTTMP/foo.hg'
+  >   saved backup bundle to \$TESTTMP/foo.hg
+  >   $ echo 'saved backup bundle to \$TESTTMP/foo.hg'
+  >   saved backup bundle to $TESTTMP\\foo.hg
+  >   $ echo 'saved backup bundle to \$TESTTMP/foo.hg'
+  >   saved backup bundle to \$TESTTMP/*.hg (glob)
+  > EOF
   $ echo 'y' | rt -i 2>&1
   
   --- $TESTTMP/test-failure.t