gen-tests: less hard-coding of 't' as the test directory

gen-tests: less hard-coding of 't' as the test directory

This will make life easier for us in case we decide to rename the
testsuite directory in the future (perhaps to something like 'tests',
which is somewhat less cryptic than the current 't').

* gen-testsuite-part ($testdir): New variable, used throughout instead
of hard-coding the name of the test directory to 't'.
All the script adjusted to make proper use of '$testdir'.
($auxdir): Move its definition earlier, for consistency with the one
of '$testdir'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>