tests: quote environment variable to extract wildcard on MinGW environment

On MinGW environment, the command line below in test script can't
extract wildcard "*" and remove target files correctly.

    $ rm $ENVVAR/foo/bar.*

To extract wildcard, environment variable should be quoted by double
quotation like below:

    $ rm "$ENVVAR"/foo/bar.*

This patch also omits "-f" of "rm" to know whether files are removed
or not by exit code of it.

tests/test-extension.t

@@ -230,7 +230,7 b' Check absolute/relative import of extens'
   $TESTTMP/a
 
 #if no-py3k
-  $ rm -f $TESTTMP/extroot/foo.*
+  $ rm "$TESTTMP"/extroot/foo.*
   $ cat > $TESTTMP/extroot/foo.py <<EOF
   > # test relative import
   > buf = []