tests: shell: fix typo in vmap_timeout test script

tests: shell: fix typo in vmap_timeout test script

While executing the test suite from tests/shell folder, the following error
is displayed many times:

tests/shell/testcases/maps/vmap_timeout: line 48: [: : integer expected

Looking at the script, a non-existing variable (expires) is tested instead of
the existing one (expire).

Reproduction:
tests/shell/run-tests.sh -v

Fixes: db80037c0279 ("tests: shell: extend vmap test with updates")
Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>