The testimony and the law refer to the scriptures, which contain the testimony of Jehovah and the law, which came from Jehovah. Those are to be the salvation of the disciples, or the faithful. This contrasts to those in the previous verses who would find the law a stumbling-block.
In verse 17, Isaiah notes that the destruction is coming, but that there is hope during, and particularly after, that destruction. Thus, the faithful will wait upon the day of the Lord, when the covenant is faithfully lived. Temporarily, the Lord is not with the house of Jacob. Isaiah uses the image of hiding his face, but that is a poetic image. Humankind might hide their faces in shame, but Jehovah’s face, or his presence, is absent because his children refuse to see him.
Verse 18 ends more hopefully. For those who wait upon the day of the Lord, there will be signs and wonders, and the Lord of Hosts will no longer be hidden from his children, but will dwell with them in Mount Zion.