fix: support relative path with drive information in Path.GetFullPath (#411)

Support relative paths with drive information in `Path.GetFullPath` as
described in
TestableIO/System.IO.Abstractions#1044.

Author: vbreuss

Source/Testably.Abstractions.Testing/FileSystem/PathMock.cs

@@ -37,6 +37,21 @@ public override string GetFullPath(string path)
 	{
 		path.EnsureValidArgument(FileSystem, nameof(path));
 
+		string? pathRoot = Path.GetPathRoot(path);
+		string? directoryRoot = Path.GetPathRoot(_fileSystem.Storage.CurrentDirectory);
+		if (!string.IsNullOrEmpty(pathRoot) && !string.IsNullOrEmpty(directoryRoot))
+		{
+			if (char.ToUpperInvariant(pathRoot[0]) != char.ToUpperInvariant(directoryRoot[0]))
+			{
+				return Path.GetFullPath(path);
+			}
+
+			if (pathRoot.Length < directoryRoot.Length)
+			{
+				path = path.Substring(pathRoot.Length);
+			}
+		}
+
 		return Path.GetFullPath(Path.Combine(
 			_fileSystem.Storage.CurrentDirectory,
 			path));

Tests/Testably.Abstractions.Tests/FileSystem/Path/GetFullPathTests.cs

@@ -5,6 +5,49 @@ public abstract partial class GetFullPathTests<TFileSystem>
 	: FileSystemTestBase<TFileSystem>
 	where TFileSystem : IFileSystem
 {
+	[SkippableFact]
+	public void GetFullPath_Dot_ShouldReturnToCurrentDirectory()
+	{
+		string expectedFullPath = FileSystem.Directory.GetCurrentDirectory();
+
+		string result = FileSystem.Path.GetFullPath(".");
+
+		result.Should().Be(expectedFullPath);
+	}
+
+	[SkippableFact]
+	public void GetFullPath_RelativePathWithDrive_ShouldReturnExpectedValue()
+	{
+		Skip.IfNot(Test.RunsOnWindows);
+
+		string currentDirectory = FileSystem.Directory.GetCurrentDirectory();
+		string drive = currentDirectory.Substring(0, 1);
+		string input = $"{drive}:test.txt";
+		string expectedFullPath = FileSystem.Path.Combine(currentDirectory, "test.txt");
+
+		string result = FileSystem.Path.GetFullPath(input);
+
+		result.Should().Be(expectedFullPath);
+	}
+
+	[SkippableFact]
+	public void
+		GetFullPath_RelativePathWithDrive_WhenCurrentDirectoryIsDifferent_ShouldReturnExpectedValue()
+	{
+		Skip.IfNot(Test.RunsOnWindows);
+
+		string currentDirectory = FileSystem.Directory.GetCurrentDirectory();
+		string otherDrive = currentDirectory
+			.Substring(0,1)
+			.Equals("x", StringComparison.OrdinalIgnoreCase) ? "Y" : "X";
+		string input = $"{otherDrive}:test.txt";
+		string expectedFullPath = $@"{otherDrive}:\test.txt";
+
+		string result = FileSystem.Path.GetFullPath(input);
+
+		result.Should().Be(expectedFullPath);
+	}
+
 	[SkippableTheory]
 	[InlineData(@"top/../most/file", @"most/file")]
 	[InlineData(@"top/../most/../dir/file", @"dir/file")]