cmd: Add test to show ~dp0 should be directory containing batch file.

programs/cmd/tests/test_builtins.cmd

@@ -14,3 +14,12 @@ echo ------------ Testing 'set' --------------
 echo set "FOO=bar" should not include the quotes in the variable value
 set "FOO=bar"
 echo %FOO%
+
+echo ------------ Testing variable expansion --------------
+echo ~dp0 should be directory containing batch file
+echo %~dp0
+mkdir dummydir
+cd dummydir
+echo %~dp0
+cd ..
+rmdir dummydir

programs/cmd/tests/test_builtins.cmd.out

@@ -12,3 +12,7 @@ at-echoed-word
 ------------ Testing 'set' --------------
 set "FOO=bar" should not include the quotes in the variable value
 bar
+------------ Testing variable expansion --------------
+~dp0 should be directory containing batch file
+@pwd@\
+@pwd@\