tests/test-empty-file.t

run-tests: set a default largefiles usercache in the default hgrc file

This fixes a test failure introduced in 4be754832829 on Windows and OS X, where
the cached largefile wasn't being deleted because the named .cache directory
didn't exist.  It only existed on Linux because the test suite sets $HOME to the
directory of the test being run, and Linux uses $HOME/.cache by default.

Most of the other largefiles tests explicitly set this value at the top of their
scripts, but test-largefiles-update.t didn't pick that up when it was created.
Those scripts that do set a value will override this.

We could just set the parameter in the test-largefiles-update.t script, but
there are a few other non obvious tests that exercise largefiles too.  These
largefiles end up being cached in the user's real cache, so proper hygiene
dictates that this not be left to each individual test script.

  $ hg init a
  $ cd a
  $ touch empty1
  $ hg add empty1
  $ hg commit -m 'add empty1'

  $ touch empty2
  $ hg add empty2
  $ hg commit -m 'add empty2'

  $ hg up -C 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ touch empty3
  $ hg add empty3
  $ hg commit -m 'add empty3'
  created new head

  $ hg heads
  changeset:   2:a1cb177e0d44
  tag:         tip
  parent:      0:1e1d9c4e5b64
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     add empty3
  
  changeset:   1:097d2b0e17f6
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     add empty2
  

  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

Before changeset 05257fd28591, we didn't notice the
empty file that came from rev 1:

  $ hg status
  M empty2
  $ hg commit -m merge
  $ hg manifest --debug tip
  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty1
  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty2
  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty3

  $ cd ..