Allow manually triggering testing workflow

This is to facilitate testing while developing new features, as the test
suite is not trivial to run on a local machine or even a fork and as
opening pull requests for all experimental features would create a lot
of noise.

.github/workflows/main.yml

@@ -1,6 +1,6 @@
 name: Test and build
 
-on: [pull_request]
+on: [pull_request, workflow_dispatch]
 
 jobs:
   build: