tests: fix a spurious failure with dash

tests: fix a spurious failure with dash

The dash shell, at least version 0.5.5.1, doesn't always bail out
with a syntax error when a stray "fi" in encountered:

  $ dash -c ":; fi"; echo stat = $?
  stat = 0

See also the relevant bug report:

  <http://permalink.gmane.org/gmane.comp.shells.dash/717>

This behaviour was causing a spurious error in our testsuite.  Fix it.

* t/self-check-exit.tap: Use a stray parentheses rather than a stray
'fi' to trigger a syntax error.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>