OSSL_STORE: don't test file: URIs on Mingw

Under a mingw shell, the command line path conversion either mangles
file: URIs to something useless (file;C:\...) or not at all (which
can't be opened by the Windows C RTL unless we're really lucky), so we
simply skip testing them in that environment.

Fixes #6369

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6376)