[se-edu#107] runtests.bat: Allow user to run script outside test fold…

…er (se-edu#108)

runtests.bat script does not work outside the test folder.

Developers working on the project often have their working directory
set to the root of the project. It is inconvenient to switch in and out
of the test folder repeatedly in an edit-debug cycle.

To reduce the hassle, let's add the commands in bat script to perform
the switching of directory automatically and hence increase our
productivity.

test/runtests.bat

@@ -1,12 +1,17 @@
 @ECHO OFF
 
+REM change current directory to the location of this script
+pushd %~dp0
+
 REM create bin directory if it doesn't exist
 if not exist ..\bin mkdir ..\bin
 
 REM compile the code into the bin folder
 javac  ..\src\seedu\addressbook\Addressbook.java -d ..\bin
 IF ERRORLEVEL 1 (
     echo ********** BUILD FAILURE ********** 
+    REM return to previous directory
+    popd
     exit /b 1
 )
 REM no error here, errorlevel == 0
@@ -28,3 +33,6 @@ java -classpath ..\bin seedu.addressbook.AddressBook < input.txt >> actual.txt
 
 REM compare the output to the expected output
 FC actual.txt expected.txt
+
+REM return to previous directory
+popd