appveyor: test OS, run os.bat in a nested cmd.exe, fix: MS-DOS was ignoring one line

appveyor:
* test the OS
* run os.bat in a nested cmd.exe as a temporary fix for #12
* fix issue with MS-DOS ignoring one line

appveyor.yml

@@ -60,8 +60,10 @@ build_script:
   - cmd: make all
 
 test_script:
-  - cmd: echo test command using batch
-  - ps: Write-Host 'or using PowerShell'
+  - cmd: cmd /C os.bat exit
+  - cmd: sh os.bat exit
+  # The next command actually runs the script as a MS-DOS batch file
+  - cmd: sh -c './os.bat exit'
 
 #artifacts:
 #  - path: path/to/file_or_directory_to_zip

example-os/os.asm

@@ -84,7 +84,10 @@ times (34*512)-($-$$) db 0
 
 ;;; End of line for MS-DOS, so that it correctly processes the GOTO instruction
 db 0x0d, 0x0a
-db "GOTO msdos", 0x0d, 0x0a
+;;; For some reason, MS-DOS seems to ignore this line, so we leave an empty comment there.
+db "::"
+db 0x0d, 0x0a
+db "@GOTO msdos", 0x0d, 0x0a
 
 ;;; After the bootsector, close the sh here-document skipped via : <<'EOF'
 db 0x0a
@@ -105,8 +108,8 @@ db "while :; do sleep 1; done", 0x0a
 db 0x0d, 0x0a
 db ":msdos", 0x0d, 0x0a
 db "@cls", 0x0d, 0x0a
-db "echo Hello world by the OS, from MS-DOS!", 0x0d, 0x0a
-db "@goto %1 :pause", 0x0d, 0x0a
+db "@echo Hello world by the OS, from MS-DOS!", 0x0d, 0x0a
+db "@GOTO %1 pause", 0x0d, 0x0a
 db ":pause", 0x0d, 0x0a
 db "@pause", 0x0d, 0x0a
 db ":exit", 0x0d, 0x0a