Fallback to process cwd when resolving drive cwd

The `path.resolve()` function when given just a drive letter such as
"C:" tries to get a drive-specific CWD, but that isn't available in
cases when the process is not launched via cmd.exe and the process
CWD has not been explicitly set on that drive.

This change adds a fallback to the process CWD, if the process CWD
happens to be on the resolved drive letter. If the process CWD is on
another drive, then a drive-specific CWD cannot be resolved and
defaults to the drive's root as before.

Based on experimentation, the fixed behavior matches that of other
similar path resolution implementations on Windows I checked: .NET's
`System.IO.Path.GetFullPath()` and Python's `os.path.abspath()`.

In the automated path test cases the issue doesn't occur when the
tests are run normally from cmd.exe. But it did cause an assertion
when running the tests from PowerShell, that is fixed by this change.

Fixes: nodejs#7215

lib/path.js

@@ -169,10 +169,12 @@ const win32 = {
       } else {
         // Windows has the concept of drive-specific current working
         // directories. If we've resolved a drive letter but not yet an
-        // absolute path, get cwd for that drive. We're sure the device is not
+        // absolute path, get cwd for that drive, or the process cwd if
+        // the drive cwd is not available. We're sure the device is not
         // a UNC path at this points, because UNC paths are always absolute.
-        path = process.env['=' + resolvedDevice];
-        // Verify that a drive-local cwd was found and that it actually points
+        path = process.env['=' + resolvedDevice] || process.cwd();
+
+        // Verify that a cwd was found and that it actually points
         // to our drive. If not, default to the drive's root.
         if (path === undefined ||
             path.slice(0, 3).toLowerCase() !==