py3: replace file() with open() in test-fileset.t

file() is not present in Python 3.
This patch also adds a b'' prefix to make sure we write bytes in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2127

tests/test-fileset.t

@@ -180,7 +180,7 b' Test files status in different revisions'
 
 Test files properties
 
-  >>> file('bin', 'wb').write('\0a')
+  >>> open('bin', 'wb').write(b'\0a')
   $ fileset 'binary()'
   $ fileset 'binary() and unknown()'
   bin
@@ -219,8 +219,8 b' Test files properties'
   $ hg --config ui.portablefilenames=ignore add con.xml
 #endif
 
-  >>> file('1k', 'wb').write(' '*1024)
-  >>> file('2k', 'wb').write(' '*2048)
+  >>> open('1k', 'wb').write(b' '*1024)
+  >>> open('2k', 'wb').write(b' '*2048)
   $ hg add 1k 2k
   $ fileset 'size("bar")'
   hg: parse error: couldn't parse size: bar