wasm: Make sure QDir::mkpath("/") works on webassembly

As described in bug QTBUG-127767 mkdir("/") returns ENOSPC
on webassembly, and the code is not prepared to handle that.
We solve it by explicitly checking for "/" which shall
always be present.

Fixes: QTBUG-127767
Change-Id: Icf86f0b681ac3bd8ddc97c1a4168c2faf02aa4a1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qfilesystemengine_unix.cpp

@@ -1078,6 +1078,11 @@ bool QFileSystemEngine::cloneFile(int srcfd, int dstfd, const QFileSystemMetaDat
 static bool createDirectoryWithParents(const QByteArray &nativeName, mode_t mode,
                                        bool shouldMkdirFirst = true)
 {
+#ifdef Q_OS_WASM
+    if (nativeName.length() == 1 && nativeName[0] == '/')
+        return true;
+#endif
+
     // helper function to check if a given path is a directory, since mkdir can
     // fail if the dir already exists (it may have been created by another
     // thread or another process)

tests/auto/corelib/io/qdir/tst_qdir.cpp

@@ -522,9 +522,6 @@ void tst_QDir::makedirReturnCode()
 
     // the next line specifically targets Windows and macOS (QTBUG-85997, QTBUG-97110)
     // calling mkpath on an existing drive name (Windows) or root path (macOS) shall pass
-#ifdef Q_OS_WASM
-    QEXPECT_FAIL("", "fails on wasm, see bug QTBUG-127767", Continue);
-#endif
     QVERIFY(QDir().mkpath(QDir::rootPath()));
     QVERIFY(!QDir().mkdir(QDir::rootPath()));