Port stdlib_io tests to test-drive

[milancurcic]

This was a fun exercise. All stdlib_io tests now run under the new testing framework. loadtxt* tests were re-written as the old tests were only printing to screen and not checking values. However, they do depend on savetxt for round-tripping the data, so neither test_savetxt and test_loadtxt are isolated from each other.

[milancurcic]

test_loadtxt fails with ifort because of the default formatting used in savetxt, which is not portable. I'll open a separate issue and patch in fortran-lang/stdlib.

[awvwgk]

The get_outpath() routine will clash with the logic used in the testing framework to select the unit test to run. At the moment this doesn't matter, but is an issue we have to fix when we start merging the suites.