[test] Add a set of default tests (emscripten-core#26391)

These are the tests that get run if you run just `./test/runner` without
any args

On my machine these run in about 2m20s:

```
$ ./test/runner 
Using default tests: jslib other core0
Running 2765 tests
Using 128 parallel test processes
[2765/2765] test_poppler (test_core.core0.test_poppler) ... ok

----------------------------------------------------------------------
Ran 2765 tests in 141.404s

OK (skipped=79)
Total core time: 12129.763s. Wallclock time: 141.404s. Parallelization: 85.78x.
```

Author: sbc100

test/runner.py

@@ -127,6 +127,8 @@
   'browser_2gb',
 ]
 
+default_tests = ['jslib', 'other', 'core0']
+
 
 def check_js_engines():
   if not all(jsrun.check_engine(e) for e in config.JS_ENGINES):
@@ -724,6 +726,10 @@ def prepend_default(arg):
 
   tests = [prepend_default(t) for t in options.tests]
 
+  if not tests:
+    errlog(f"Using default tests: {' '.join(default_tests)}")
+    tests = default_tests
+
   modules = get_and_import_modules()
   all_tests = get_all_tests(modules)
   if options.crossplatform_only:
@@ -734,10 +740,6 @@ def prepend_default(arg):
     tests = skip_requested_tests(tests, modules)
     tests = args_for_random_tests(tests, modules)
 
-  if not tests:
-    errlog('ERROR: no tests to run')
-    return 1
-
   if not options.start_at and options._continue:
     if os.path.exists(common.LAST_TEST):
       options.start_at = utils.read_file(common.LAST_TEST).strip()