test: Convert Window path to MSYS path

When MSYS shell executes program, if its arguments look like MSYS paths,
MSYS automatically converts them into Windows paths.
For example, `/c/path:/d/path` becomes `C:\path;D:\path`.
However, if there is only one path e.g. `/c/path`, it becomes `C:/path`.

maketest.py reverts the behavior to reduce confusion between MSYS and
Windows, but it didn't handle the `/c/path` case. This patch fixes the
issue.

Fixes #15297
Fixes #15250