Changes

[DWARFYAML][test] Make the checker stricter. NFC.

Currently, DWARFYAML doesn't emit the 0 byte for terminating the abbrev
table for the given compilation unit. Before addressing this issue, we
have to make the test stricter.

[NewPM] Add explicit init value to -enable-new-pm

 So it's easier to test with it on by default.

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D82922