Bug 1062228 - Set re.IGNORECASE correctly for the build_type regex

Only the re module function .search() accepts the flags argument, for
the regexobject search() method it actually gets interpreted as the
'pos' parameter, since |re.IGNORECASE == 2|.

With this change we now correctly identify tests run on ASAN builds as
buildtype 'asan'.
Merge pull request #198 from mozilla/buildtype

Set re.IGNORECASE correctly for the build_type regex (bug 1062228)