Add self test, drop external test trigger by push

Triggering external repo's test requires to maintain auth token.
We might want to run it occasionally, but it doesn't need to be
on every push.  So we run test inside repo on push.
External repo test can be manually triggered when necessary.

.github/workflows/main.yml

@@ -1,6 +1,6 @@
 name: Trigger setup-gauche-tester
 
-on: [ push, workflow_dispatch ]
+on: [ workflow_dispatch ]
 
 jobs:
   trigger-tester:

.github/workflows/self-test.yml

@@ -0,0 +1,42 @@
+name: Run setup-gauche action
+
+on: [ push ]
+
+
+# We don't use cross-matrix for platform and gauche-version, for
+# we want to test specifically setup-gauche _without_ parameters
+# and _with_ parameters.
+
+jobs:
+  test-setup-latest:
+    strategy:
+      matrix:
+        platform: [ubuntu-latest, macos-12]
+    runs-on: ${{ matrix.platform }}
+    steps:
+    - uses: shirok/setup-gauche@main
+    - name: Run gosh
+      run: gosh -V
+
+  test-setup-snapshot:
+    strategy:
+      matrix:
+        platform: [ubuntu-latest, macos-12]
+    runs-on: ${{ matrix.platform }}
+    steps:
+    - uses: shirok/setup-gauche@main
+      with:
+        gauche-version: 'snapshot'
+        test-gauche: true
+        configure-options: '--with-slib=/opt'
+    - name: Run gosh
+      run: gosh -V
+
+  test-setup-binary:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: shirok/setup-gauche@main
+      with:
+        prebuilt-binary: true
+    - name: Run gosh
+      run: gosh -V