test: set VG_TEST_ARGS correctly.

Tests should not leak memory.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
test: report reachable leaks.

Valgrind doesn't exit with an error for reachable leaks, but we want to
catch them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>