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 rust-lang#15297
Fixes rust-lang#15250

src/etc/maketest.py

@@ -15,13 +15,14 @@
 # msys1/msys2 automatically converts `/abs/path1:/abs/path2` into
 # `c:\real\abs\path1;c:\real\abs\path2` (semicolons) if shell thinks
 # the value is list of paths.
+# (if there is only one path, it becomes `c:/real/abs/path`.)
 # this causes great confusion and error: shell and Makefile doesn't like
 # windows paths so it is really error-prone. revert it for peace.
 def normalize_path(v):
-    # c:\path -> /c/path
-    if ':\\' in v:
-        v = '/' + v.replace(':\\', '/')
     v = v.replace('\\', '/')
+    # c:/path -> /c/path
+    if ':/' in v:
+        v = '/' + v.replace(':/', '/')
     return v