cmd/tests: Test that the if command is not influenced by a previous one.

programs/cmd/tests/test_builtins.cmd

@@ -1013,6 +1013,13 @@ if /i not (a)==(b) (
 ) else (
   echo comparison operators surrounded by brackets seem to be broken
 )
+if defined windir echo windir is defined
+if not defined windir echo windir is defined
+if not exist %windir% (
+  echo windir does not exist
+) else (
+  echo windir does exist
+)
 echo --- case sensitivity with and without /i option
 if bar==BAR echo if does not default to case sensitivity
 if not bar==BAR echo if seems to default to case sensitivity

programs/cmd/tests/test_builtins.cmd.exp

@@ -699,6 +699,8 @@ comparison operators surrounded by brackets seem to work
 comparison operators surrounded by brackets seem to work
 comparison operators surrounded by brackets seem to work
 comparison operators surrounded by brackets seem to work
+windir is defined
+windir does exist
 --- case sensitivity with and without /i option
 if seems to default to case sensitivity
 if /i seems to work