tests: put temporary file outside the working directory for test portability

test-largefiles-update.t creates temporary file exec-bit.patch inside
the working directory for no-execbit platform specific test, but
subsequent tests aren't aware of it.

On execbit platform, subsequent tests can run successfully, because
exec-bit.patch isn't created.

But on no-execbit platform, this temporary file makes subsequent tests
show "? exec-bit.patch" at each "hg status".

tests/test-largefiles-update.t

@@ -712,7 +712,7 b' bit correctly on the platform being unaw'
 
   $ hg update -q -C 4
 
-  $ cat > exec-bit.patch <<EOF
+  $ cat > ../exec-bit.patch <<EOF
   > # HG changeset patch
   > # User test
   > # Date 0 0
@@ -725,8 +725,8 b' bit correctly on the platform being unaw'
   > old mode 100644
   > new mode 100755
   > EOF
-  $ hg import --exact --bypass exec-bit.patch
-  applying exec-bit.patch
+  $ hg import --exact --bypass ../exec-bit.patch
+  applying ../exec-bit.patch
   $ hg status -A --rev tip large2
   C large2