Fix the mischecking xml output in system tests and refactor the checking process

[bbell]

Update: There was a problem with system test, when output especially in xml always return true from validation. The systemtest need to be fixed.

run test.sh in systemtest/testcases_mandatory/test_nullvalues/test.sh
first time —> everything passes
second time —> encode.rasql fails
third time —> encode.rasql fails
fourth time —> everything passes

After some investigation, the behaviour in lines 839-892 of systemtest/util/common.sh creates redundantly-named output files which conflict with what is being tested on subsequent tests.

Compare lines 854-869 with lines 875-890. The former, which is for *.tiff files, uses $output_tmp, while the latter uses 2>&1 > /dev/null. Could this discrepancy be what is causing the errors in the tests? Perhaps removing the files after comparison could resolve this issue.