Browser tests try to create a valid URL to set in the browser. The
mechanism to determine this URL depends on whether the test is executed
in an Ant build. This mechanism uses an unnecessary conversion to a URI,
which can fail in case the URL string is an OS-specific absolute file
system path.

This change removes the unnecessary conversion and a code duplication.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.swt/issues/1455