Extend bilingual_str support for tinyformat

Previous bilingual_str tinyformat::format accepted bilingual format strings,
but not bilingual arguments. Extend it to accept both. This is useful when
embedding one translated string inside another translated string, for example:
`strprintf(_("Error: %s"), message)` which would fail previously if `message`
was a bilingual_str.

test/lint/run-lint-format-strings.py

@@ -17,6 +17,7 @@ FALSE_POSITIVES = [
     ("src/index/base.cpp", "FatalError(const char* fmt, const Args&... args)"),
     ("src/netbase.cpp", "LogConnectFailure(bool manual_connection, const char* fmt, const Args&... args)"),
     ("src/clientversion.cpp", "strprintf(_(COPYRIGHT_HOLDERS).translated, COPYRIGHT_HOLDERS_SUBSTITUTION)"),
+    ("src/test/translation_tests.cpp", "strprintf(format, arg)"),
     ("src/validationinterface.cpp", "LogPrint(BCLog::VALIDATION, fmt \"\\n\", __VA_ARGS__)"),
     ("src/wallet/wallet.h",  "WalletLogPrintf(std::string fmt, Params... parameters)"),
     ("src/wallet/wallet.h", "LogPrintf((\"%s \" + fmt).c_str(), GetDisplayName(), parameters...)"),