Fix unit tests to enable to run NullCoalescingTest.php and TernaryTest.php on Windows

[matsuo]

This PR fixes unit tests to enable to run NullCoalescingTest.php and TernaryTest.php on Windows. (Related PR: #1046 )

Current Test Results:
(from #1046 (comment) )

45) NullCoalescingTest::testUndefined
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\NullCoalescingTest.php:7

46) NullCoalescingTest::testOther with data set #0 (null, 'undefined')
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\NullCoalescingTest.php:7

47) NullCoalescingTest::testOther with data set #1 ('blah', 'blah')
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\NullCoalescingTest.php:7

48) NullCoalescingTest::testOther with data set #2 ('', '')
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\NullCoalescingTest.php:7

49) NullCoalescingTest::testOther with data set #3 (false, false)
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\NullCoalescingTest.php:7

50) TernaryTest::testTernaryTrue
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\TernaryTest.php:7

51) TernaryTest::testTernaryFalse
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\TernaryTest.php:7

52) TernaryTest::testShorthandTernaryTrue
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\TernaryTest.php:7

53) TernaryTest::testShorthandTernaryFalse
chdir(): No such file or directory (errno 2)

D:\a\smarty\smarty\tests\PHPUnit_Smarty.php:107
D:\a\smarty\smarty\tests\UnitTests\TemplateSource\TernaryTest.php:7