Introduce/improve check_js_errors and improve test_no_implicit_target (#874)

Until now, we didn't have a nice way to check that we expect a specific JS error in the web page.
This PR improves check_js_errors() so that now you can pass a list of error messages that you expect.
It is tricky because we need to handle (and test!) all various combinations of cases:

- errors expected and found / expected but not found
- unexpected errors found / not found

Moreover, JS exceptions now are logged in the special category console.js_error, which means that the printed text is also available using e.g. self.console.js_error.text or self.console.all.text. However, this should never be required and it's preferred to use self.check_js_errors to check for exceptions. This fixes #795 .

Finally, use the new logic to improve test_no_implicit_target.

pyscriptjs/tests/integration/test_py_config.py

@@ -5,7 +5,7 @@ import tempfile
 import pytest
 import requests
 
-from .support import JsError, PyScriptTest
+from .support import PyScriptTest
 
 URL = "https://github.com/pyodide/pyodide/releases/download/0.20.0/pyodide-build-0.20.0.tar.bz2"
 TAR_NAME = "pyodide-build-0.20.0.tar.bz2"
@@ -105,29 +105,32 @@ class TestConfig(PyScriptTest):
         assert version == "0.20.0"
 
     def test_invalid_json_config(self):
-        with pytest.raises(JsError) as exc:
-            self.pyscript_run(
-                snippet="""
-                <py-config type="json">
-                    [[
-                </py-config>
-                """
-            )
-
-        msg = str(exc.value)
-        assert "SyntaxError" in msg
+        # we need wait_for_pyscript=False because we bail out very soon,
+        # before being able to write 'PyScript page fully initialized'
+        self.pyscript_run(
+            """
+            <py-config type="json">
+                [[
+            </py-config>
+            """,
+            wait_for_pyscript=False,
+        )
+        self.page.wait_for_selector(".py-error")
+        self.check_js_errors("Unexpected end of JSON input")
 
     def test_invalid_toml_config(self):
-        with pytest.raises(JsError) as exc:
-            self.pyscript_run(
-                snippet="""
-                <py-config>
-                    [[
-                </py-config>
-                """
-            )
-        msg = str(exc)
-        assert "<ExceptionInfo JsError" in msg
+        # we need wait_for_pyscript=False because we bail out very soon,
+        # before being able to write 'PyScript page fully initialized'
+        self.pyscript_run(
+            """
+            <py-config>
+                [[
+            </py-config>
+            """,
+            wait_for_pyscript=False,
+        )
+        self.page.wait_for_selector(".py-error")
+        self.check_js_errors("SyntaxError: Expected DoubleQuote")
 
     def test_multiple_py_config(self):
         self.pyscript_run(