Test: remove bogus argument from prove invocation

An argument of "$2" was added to the prove command line in tests/run.sh
by 68270f0f604e "Mi test: Basic test structure". This is harmless since
run.sh is never called with more than one argument, but it is confusing,
and it breaks patches in OE that add support for running tests on the
target.

Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>