refactor(expect): also capture EOF when using expectation helpers

In the OpenCI there's no possibility of a timeout when running Expect
scripts, but there can be an EOF. This change modifies the
`expect_string` and `expect_re` functions to report that a match was not
found when an early EOF is detected.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I12fbfc86456d20e058f46b388f389d057b9d734c
1 file changed