The xref tests fail on case sensitive filesystems if the source directory
is not all lower case.

For example, if I have Emacs checked out in /home/wilfred/src/EMACS, I get
the following test failure:

Test xref-elisp-test-find-defs-defalias-defun-el condition:
    (error "Can't find library /home/wilfred/src/emacs/lisp/buff-menu.el")
   FAILED  12/36  xref-elisp-test-find-defs-defalias-defun-el

Note that the path is incorrectly converted from ~/src/EMACS to
~/src/emacs, which doesn't exist.

This causes the following tests to fail:

   FAILED  xref-elisp-test-find-defs-constructor
   FAILED  xref-elisp-test-find-defs-defalias-defun-el
   FAILED  xref-elisp-test-find-defs-defgeneric-co-located-default
   FAILED  xref-elisp-test-find-defs-defgeneric-el
   FAILED  xref-elisp-test-find-defs-defgeneric-implicit-generic
   FAILED  xref-elisp-test-find-defs-defgeneric-no-default
   FAILED  xref-elisp-test-find-defs-defgeneric-no-methods
   FAILED  xref-elisp-test-find-defs-defgeneric-separate-default
   FAILED  xref-elisp-test-find-defs-define-overload-co-located-default
   FAILED  xref-elisp-test-find-defs-define-overload-no-default
   FAILED  xref-elisp-test-find-defs-define-overload-no-methods
   FAILED  xref-elisp-test-find-defs-define-overload-separate-default
   FAILED  xref-elisp-test-find-defs-defun-defvar-el
   FAILED  xref-elisp-test-find-defs-defun-el
   FAILED  xref-elisp-test-find-defs-defun-el-defvar-c
   FAILED  xref-elisp-test-find-defs-defvar-el
   FAILED  xref-elisp-test-find-defs-face-el
   FAILED  xref-elisp-test-find-defs-feature-el