syn2mas: Add tests for reading and writing threepids #1697

Job	Run time
Run `cargo deny` checks	26s
Check schema	4m 6s
Check the frontend for unused dependencies	26s
Compile test artifacts	4m 4s
Check frontend style	31s
Run the frontend test suite	35s
Check Rust style	11s
Lint and test OPA policies	4s
Check syn2mas	17s
Run Clippy	2m 15s
Run test suite with Rust stable (1)	1m 54s
Run test suite with Rust stable (2)	1m 48s
Run test suite with Rust stable (3)	1m 49s
Tests done	4s
	18m 30s

Provide feedback