exec-test: Work around Bash 3.2.57 ERR trap bug

When running under Bash 3.2.57(1)-release on macOS, the following tests
would fail because `BATS_ERROR_STACK_TRACE` would be empty, and hence no
information about the actual error would get printed:

- one failing test
- failing test with significant status
- failing test file outside of BATS_CWD

This is because each of these cases use `FIXTURE_ROOT/failing.bats`, and
the `ERR` trap would not fire for its `eval "( exit ${STATUS:-1} )"`
line. Changing it to `exit ${STATUS:-1}` produced the same effect, and
changing it to `return ${STATUS:-1}` would cause the output to point to
the previous line, which executes `true`.

However, the correct status would be reported to the `EXIT` trap, so now
we call `bats_error_trap` at the very beginning of `bats_teardown_trap`.

All the existing tests now pass under Bash 3.2.57(1)-release, Bash
4.2.25(1)-release (the version from the default Ubuntu 12.04.5/Precise
image on Travis CI), and Bash 4.4.12(1)-release.

libexec/bats-exec-test

@@ -218,13 +218,22 @@ bats_debug_trap() {
   fi
 }
 
+# When running under Bash 3.2.57(1)-release on macOS, the `ERR` trap may not
+# always fire, but the `EXIT` trap will. For this reason we call it at the very
+# beginning of `bats_teardown_trap` (the `DEBUG` trap for the call will move
+# `BATS_CURRENT_STACK_TRACE` to `BATS_PREVIOUS_STACK_TRACE`) and check the value
+# of `$?` before taking other actions.
 bats_error_trap() {
-  BATS_ERROR_STATUS="$?"
+  local status="$?"
+  if [[ "$status" -ne '0' ]]; then
+    BATS_ERROR_STATUS="$status"
     BATS_ERROR_STACK_TRACE=( "${BATS_PREVIOUS_STACK_TRACE[@]}" )
     trap - debug
+  fi
 }
 
 bats_teardown_trap() {
+  bats_error_trap
   trap "bats_exit_trap" exit
   local status=0
   teardown >>"$BATS_OUT" 2>&1 || status="$?"