Make MAX_NAME_BYTES on WASI equivalent to the max of the other platforms

Make the test use the minimum length and set MAX_NAME_BYTES to the maximum so that:
- the test will work on any host platform
- *and* the MAX_NAME_BYTES will be able to hold the max file name component on any host platform

lib/std/fs.zig

@@ -59,8 +59,10 @@ pub const MAX_NAME_BYTES = switch (builtin.os.tag) {
     // If it would require 4 UTF-8 bytes, then there would be a surrogate
     // pair in the UTF-16LE, and we (over)account 3 bytes for it that way.
     .windows => os.windows.NAME_MAX * 3,
-    // TODO work out what a reasonable value we should use here
-    .wasi => 255,
+    // For WASI, the MAX_NAME will depend on the host OS, so it needs to be
+    // as large as the largest MAX_NAME_BYTES in order to work on any host OS.
+    // TODO determine if this is a reasonable approach
+    .wasi => os.windows.NAME_MAX * 3,
     else => if (@hasDecl(root, "os") and @hasDecl(root.os, "NAME_MAX"))
         root.os.NAME_MAX
     else

lib/std/fs/test.zig

@@ -735,6 +735,11 @@ test "filename limits" {
         // so Windows allows for NAME_MAX of them
         const maxed_windows_filename = ("€".*) ** std.os.windows.NAME_MAX;
         try testFilenameLimits(tmp.iterable_dir, &maxed_windows_filename);
+    } else if (builtin.os.tag == .wasi) {
+        // On WASI, the maxed filename depends on the host OS, so in order for this test to
+        // work on any host, we need to use a length that will work for all platforms.
+        const maxed_wasi_filename = [_]u8{'1'} ** 255;
+        try testFilenameLimits(tmp.iterable_dir, &maxed_wasi_filename);
     } else {
         const maxed_ascii_filename = [_]u8{'1'} ** std.fs.MAX_NAME_BYTES;
         try testFilenameLimits(tmp.iterable_dir, &maxed_ascii_filename);