test: remove redundant code in test

A block of asserts were duplicated in
test/simple/test-child-process-spawn-typeerror.js. This commit
removes the duplicated asserts.

Fixes: https://github.com/joyent/node/pull/8454
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Merge remote-tracking branch 'upstream/v0.10' into v0.10