Set fixed precision avoid rounding related issues while executing the unit tests

Fixes issues like:

```
1) JHtmlNumberTest::testBytes with data set #10 ('1099511627776 kb', 1125899906842624, 'kb')
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'1099511627776 kb'
+'1.09951162778E+12 kb'

/Users/sniper/Zend/workspaces/DefaultWorkspace/joomla-cms/tests/unit/suites/libraries/cms/html/JHtmlNumberTest.php:136

2) JHtmlNumberTest::testBytes with data set #11 ('1.1258999068426E+15 b', 1125899906842624, 'b')
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'1.1258999068426E+15 b'
+'1.12589990684E+15 b'
```