Tests: tighten shell test checks

Tests: tighten shell test checks

Previously, Expect treated timeouts as successful matches, and EOF
passed even if the REPL exited abnormally.  Some checks could match
the input echo alone, while completion checks required a bell that
varies between libedit versions.

The fix is to fail on timeouts, premature EOF, and abnormal exit;
check the result and prompt for Unicode input and guard against a
repeated promise rejection.  Send Unicode as ASCII escapes to work
around libedit 3.1.snap20150325-2.12 on SLES 15 SP7, which reports
a character count where the readline callback expects a byte count.