test-bad-extension: avoid printing the file name

This should make it simpler on windows

tests/test-bad-extension

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-echo 'syntax error' > badext.py
+echo 'raise Exception("bit bucket overflow")' > badext.py
 abspath=`pwd`/badext.py
 
 echo '[extensions]' >> $HGRCPATH

tests/test-bad-extension.out

@@ -1,4 +1,4 @@
-*** failed to import extension badext: invalid syntax (badext.py, line 1)
+*** failed to import extension badext: bit bucket overflow
 extension 'hgext.gpg' overrides commands: sigs sigcheck sign
 hg help [COMMAND]