tests/common/134: Rename copy.py to copyfile.py

On some distros, running this causes Python to find the file itself as
the implementation of the `copy` module:

$ python3 copy.py
Traceback (most recent call last):
  File "copy.py", line 4, in <module>
    import shutil
  File "/usr/lib/python3.4/shutil.py", line 14, in <module>
    import tarfile
  File "/usr/lib/python3.4/tarfile.py", line 48, in <module>
    import copy
  File "/c/Users/nirbheek/projects/meson.git/test cases/common/134 generated llvm ir/copy.py", line 6, in <module>
    shutil.copyfile(sys.argv[1], sys.argv[2])
AttributeError: 'module' object has no attribute 'copyfile'

test cases/common/134 generated llvm ir/meson.build

@@ -4,7 +4,7 @@ if meson.get_compiler('c').get_id() != 'clang'
   error('MESON_SKIP_TEST: LLVM IR files can only be built with clang')
 endif
 
-copy = find_program('copy.py')
+copy = find_program('copyfile.py')
 
 copygen = generator(copy,
   arguments : ['@INPUT@', '@OUTPUT@'],