test-strip-narrow: adjust bundle removal for Windows test stability

MSYS was mangling $TESTTMP to C:\\Users\\...\\test-narrow-strip.t-flat/, which
caused `rm` to fail.  The -f was suppressing -ENOENT, so the only clue something
was wrong was when 2 bundles were applied via `hg unbundle` on line 91, instead
of just 1.  This changed the text output of `hg unbundle`.

The first `rm` wasn't causing an issue, but is changed for consistency with the
rest of the file.

tests/test-narrow-strip.t

@@ -55,7 +55,7 b' Can strip and recover changesets affecti'
 
   $ hg co -r 'desc("modify inside")'
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
-  $ rm -f $TESTTMP/narrow/.hg/strip-backup/*-backup.hg
+  $ rm -f .hg/strip-backup/*-backup.hg
   $ hg strip .
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   saved backup bundle to $TESTTMP/narrow/.hg/strip-backup/*-backup.hg (glob)
@@ -84,7 +84,7 b' Can strip and recover changesets affecti'
   inside\x006a8bc41df94075d501f9740587a0c0e13c170dc5t (esc) (tree !)
   outside\x00255c2627ebdd3c7dcaa6945246f9b9f02bd45a09t (esc) (tree !)
 
-  $ rm -f $TESTTMP/narrow/.hg/strip-backup/*-backup.hg
+  $ rm -f .hg/strip-backup/*-backup.hg
   $ hg strip .
   0 files updated, 0 files merged, 0 files removed, 0 files unresolved
   saved backup bundle to $TESTTMP/narrow/.hg/strip-backup/*-backup.hg (glob)