test-driver.scm: Don't guess script name from "--test-name"

test-driver.scm: Don't guess script name from "--test-name"

'primitive-load' is used instead of 'load-from-path' since the script is
given as a relative file name.  For unknown reason, using 'load' fails
with GNU Mcron test suite when running 'make distcheck'.

* contrib/test-driver.scm: Get the actual script name directly from the
command line.  Handle the case where that argument is missing.