On macOS hosts, autodetect the system Safari browser (#26897)

On macOS hosts, autodetect the system Safari browser, if no other test
browser is present. This is symmetric to looking up Firefox/Chrome in
PATH on other OSes.

Fixes `emrun.test_no_browser` test on macOS when no EMTEST_BROWSER is
set.

Author: juj

test/browser_common.py

@@ -339,8 +339,11 @@ def configure_test_browser():
     if not shutil.which(EMTEST_BROWSER):
       EMTEST_BROWSER = 'firefox'
       if not shutil.which(EMTEST_BROWSER):
-        # FIXME: This should really be and error, but this code currently also runs for non-browser tests.
-        EMTEST_BROWSER = 'default-browser-not-found'
+        if MACOS and os.path.isdir('/Applications/Safari.app'):
+          EMTEST_BROWSER = '/Applications/Safari.app'
+        else:
+          # FIXME: This should really be and error, but this code currently also runs for non-browser tests.
+          EMTEST_BROWSER = 'default-browser-not-found'
 
   if WINDOWS and '"' not in EMTEST_BROWSER and "'" not in EMTEST_BROWSER:
     # On Windows env. vars canonically use backslashes as directory delimiters, e.g.