Add assert_int_equal() shell function

Add a shell function which is used in the "tcp" system test, but has
been accidentally omitted from !2425.  Make sure the function does not
change the value of "ret" itself, so that the caller can decide what to
do with the function's return value.

(cherry picked from commit 8bb7f1f2)
62 jobs for !2539 with 1206-add-assert_int_equal-shell-function-v9_14 in 14 seconds (queued for 6 seconds)
detached
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