Fix tests under MSYS2 (Windows)

Fix tests under MSYS2 (Windows)

This is a follow-up to commit 7cd748e considering running the tests
in Windows under MSYS2. A similar approach has already been used in
tests\graph2 and tests\xport1 (see also: 847d679)

- Updates the following tests:
  graph1, rpn1, rpn2, vformatter1
- Avoid Windows drive letter in the path of DEF:vname=rrdfile
  under MSYS or MSYS2. Use only the filename of the rrd and not
  the full path if environmental variable MSYSTEM is set.