ci: make required checks passed if skipped (#37)

Fix #22 

A [limitation of GitHub
actions](https://github.com/orgs/community/discussions/13690) is that ci
jobs that used as checks required to allow a PR to merge, are **always**
required, even if they were skipped.
In the case they were skipped...well the ci check isn't passed, not
allowing a valid PR to merge.

A [workaround](https://stackoverflow.com/a/77066140/9771158) is to use
`${{ !(failure() || cancelled()) }}` as a condition, meaning: if the
dependents jobs didn't fail or weren't cancelled, execute (especially:
if dependents jobs were skipped, execute).
This can be used as an `if` condition in a dummy job that will always be
executed and successful, **unless the dependents jobs failed or were
cancelled**. Which is what we want.

The [workflow
diagram](https://github.com/privacy-scaling-explorations/zk-kit.solidity/actions/runs/10297770771)
illustrates well what is going on:

![image](https://github.com/user-attachments/assets/83d4728a-ee5f-45eb-9a40-ea7faf2e4bbe)

This PR does not affect any `sol` files, so the `_tests` and `_slither`
matrix jobs are all skipped.
However `tests` and `slither` which depends on `_tests` and `_slither`
respectively and are used as ci required checks, are still executed and
pass, allowing the PR to merge.


This will speed up even more the CI, and avoid ci checks being a stuck
with `waiting for status to be reported` state.

.github/workflows/main.yml

@@ -45,7 +45,7 @@ jobs:
               id: changed-files
               uses: tj-actions/changed-files@v44
               with:
-                  files: packages/**/*.{sol,json,ts}
+                  files: packages/**/*.{json,sol,ts}
 
     compile:
         if: needs.changed-files.outputs.any_changed == 'true'
@@ -83,9 +83,10 @@ jobs:
                   key: ${{ needs.deps.outputs.cache-key }}
 
             - run: yarn format
-    tests:
+
+    _tests:
         if: needs.changed-files.outputs.any_changed == 'true'
-        needs: [changed-files, set-matrix, deps, compile]
+        needs: compile
         runs-on: ubuntu-latest
         strategy:
             matrix:
@@ -119,6 +120,16 @@ jobs:
                   parallel: true
                   flag-name: run ${{ join(matrix.*, '-') }}
 
+    tests:
+        needs: _tests
+        # workaround for https://github.com/orgs/community/discussions/13690
+        # https://stackoverflow.com/a/77066140/9771158
+        if: ${{ !(failure() || cancelled()) }}
+        runs-on: ubuntu-latest
+        steps:
+            - name: Tests OK (passed or skipped)
+              run: true
+
     set-matrix:
         if: needs.changed-files.outputs.any_changed == 'true'
         needs: changed-files
@@ -133,7 +144,7 @@ jobs:
                   matrix=$(ls -1 packages | jq -Rsc 'split("\n") | map(select(length > 0))')
                   echo "matrix=$matrix" >> $GITHUB_OUTPUT
 
-    slither:
+    _slither:
         if: needs.changed-files.outputs.any_changed == 'true'
         needs: [changed-files, set-matrix, deps]
         runs-on: ubuntu-latest
@@ -194,3 +205,11 @@ jobs:
                       const header = '# Slither report'
                       const body = process.env.REPORT
                       await script({ github, context, header, body })
+
+    slither:
+        needs: _slither
+        if: ${{ !(failure() || cancelled()) }}
+        runs-on: ubuntu-latest
+        steps:
+            - name: Slither analysis OK (passed or skipped)
+              run: true