Is China selling U.S Government Bonds (Treasury Bills, T-Bonds and Notes) given the trade war tensions between China and the United States? The simple answer to that is no. Actually, no major foreign country holder of bonds is really selling.
But you might wonder what is going on if you make a chart look like this,

Which if properly scaled looks like this,

Notably though – Russian holdings of U.S. Government bonds declined 84% between March and May 2018, falling to $14.9 billion from $96.1 billion in just two months.
China and Japan hold more than $1 trillion each in Treasuries on their books, here is a table of holdings for the past year,

Related:
Can the U.S. government really cope with rising bond yields?