Tesla Inc. reveals that its production plan for its new electric Semi heavy-duty truck remains in a low gear because the special battery cells and the large rig requirements are short in supply.The Silicon Valley car maker said this week that it plans to deliver “our first Tesla Semi by the end of the year.” Boosting production to meet that goal will test Tesla’s ability to scale and to meet pledges to fleet owners in an industry more focused on cost and performance than the typical Tesla buyer. Revealed in 2017, the Semi was initially promised to be in client hands in 2019. Its arrival has since been pushed back two years, delays that Mr. Musk has repeatedly attributed to battery-cell supply constraints.