PathFragment: test Windows mixed separator

[sluongng]

On Windows, if a user were to specify a PathFragment with mixed
separator such as

bazel build --profile='C:/foo\\bar' //...

then the profile file will be written as bar and recorded in
BuildToolLogs event as so.

Provide a test that demonstrates the current behavior to discuss whether
this is "correct" or not.

[tjgq]

I'm pretty sure that treating both / and \ as separators on Windows is intentional (see the implementation of WindowsOsPathPolicy.normalize). The backslashes are also canonicalized to forward slashes (i.e., you always get forward slashes when converting a PathFragment back to a String).

In terms of test coverage: instead of adding to testBasenameWindows, I'd suggest a separate test case verifying that the entire path is correctly broken down into segments.

[sluongng]

In terms of test coverage: instead of adding to testBasenameWindows, I'd suggest a separate test case verifying that the entire path is correctly broken down into segments.

I added some more cases with mixed usage of separators to the segment tests.