Improve tests for #17171

While backporting MR !1806 (closed) to 8.8.2 (!1885 (closed)) I learnt the following:

• Tests with expect_fail do not compare *.stderr output files. So a test using expect_fail will not detect future regressions on the stderr output.
• To compare the *.stderr output files, I have to use the exit_code(n) function.
• When a release is made, tests with makefile_test are converted to use run_command.
• For the test T17171a the return code is 1 when running makefile_test, however it's 2 when running run_command.

Therefore I decided:

• To improve my tests for #17171 (closed)
• To change test T17171a from expect_fail to exit_code(2)
• To change both tests from makefile_test to run_command