Don't require WX_TEST_WEBREQUEST_URL environment variable to be set, but
only allow defining it to override the default https://httpbin.org value
or to disable the tests by setting it to 0.
They're not necessary to use this class and we may consider exporting
them later, possibly with a better API and more tests, if really needed.
Also do change their API slightly by leaving only a single function and
returning the value instead of using an out parameter for it to make it
simpler to use.
Check that the stream is valid, if specified at all, and return false if
it isn't -- or if no size was specified and determining stream size
failed.
Check for SetData() success in the test to provide better diagnostics in
case the file it uses is not found (as is the case when running the test
from another directory, for example).
Also pass wxSharedPtr<> by const reference instead of by value to avoid
unnecessary copies.
