Switch to osemgrep test --experimental (from 3min to 21s) (#3472)

* Switch to osemgrep test --experimental

test plan:
wait for green CI check

* comment

* comments

.github/workflows/semgrep-rules-test-develop.yml

@@ -1,3 +1,6 @@
+# Running the tests in the repo using `semgrep test --experimental` and
+# the semgrep/semgrep:pro-develop docker image (the bleeding edge!).
+
 name: semgrep-rules-test-develop
 on:
   pull_request:
@@ -9,26 +12,27 @@ on:
     - develop
     - release
 jobs:
-  # Note: if you change this test there will likely need to be a
-  # corresponding change in returntocorp/semgrep
   test-develop:
     name: rules-test-develop
     runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
       with:
         path: semgrep-rules
-    - name: delete stats directory
-      run: rm -rf semgrep-rules/stats
-    - name: delete fingerprints directory
-      run: rm -rf semgrep-rules/fingerprints
+    - name: delete directories not containing rules
+      run: rm -rf semgrep-rules/stats semgrep-rules/fingerprints
+    #TODO: in theory we could run those tests by using --test --pro
+    # since our docker image now contains semgrep-core-pro (but
+    # it would require to be logged in though via a SEMGREP_APP_TOKEN)
     - name: delete rules requiring Semgrep Pro
       run: rm -rf semgrep-rules/apex semgrep-rules/elixir
+    # TODO: this takes 1m20 in CI and could be optimized by switching to osemgrep
     - name: validate rules
       run: |
-        export SEMGREP="docker run --rm -w /src -v ${GITHUB_WORKSPACE}/semgrep-rules:/src returntocorp/semgrep:develop semgrep"
+        export SEMGREP="docker run --rm -w /src -v ${GITHUB_WORKSPACE}/semgrep-rules:/src semgrep/semgrep:pro-develop semgrep"
         make -C "$GITHUB_WORKSPACE"/semgrep-rules validate
-    - name: test with semgrep develop branch
+    # this now takes 21s with osemgrep instead of 3min with pysemgrep
+    - name: test with semgrep pro develop branch and with --experimental
       run: |
-        export SEMGREP="docker run --rm -w /src -v ${GITHUB_WORKSPACE}/semgrep-rules:/src returntocorp/semgrep:develop semgrep"
+        export SEMGREP="docker run --rm -w /src -v ${GITHUB_WORKSPACE}/semgrep-rules:/src semgrep/semgrep:pro-develop semgrep --experimental"
         make -C "$GITHUB_WORKSPACE"/semgrep-rules test-only