Support pattern modifiers in the filter command line option.

Example:
The following will run tests which match the text
'simple' regardless of cases.
--filter /simple/i basicUnitTest.php

Added two tests.
Fixed one inconsistency in an existing filter test.

Currently phpunit --filter option doesn't support case insensitive filtering by specifying the 'i' pattern modifier.

Case insensitive filtering can be helpful in some situations.
For example:
A method is named 'foo'.
The corresponding test method is named 'testFoo'.
When we want to filter tests that test the method 'foo', we will need to specify the filter as --filter Foo instead of a more natural filter of the exact method name as --filter foo.
This limitation makes some automation code to invoke phpunit tests based on the test method selected in an IDE more difficult.

Currently when 'phpunit --filter /foo/i aTest.php' is executed for example, the filter is changed to '/\/foo\/i/' i.e. the code thinks that no regular expression delimiter is given.

This change adds support for case insensitive filtering by properly detecting that a pattern modifier is given.

Example:
The following
phpunit --filter /foo/i aTest.php
will match a test method named 'testFoo'.