cmdlineTests: Allow providing content of the standard input when not testing via Standard JSON

test/cmdlineTests/stdin/args

@@ -0,0 +1 @@
+-

test/cmdlineTests/stdin/err

@@ -0,0 +1,5 @@
+Warning: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.
+--> <stdin>
+
+Warning: Source file does not specify required compiler version!
+--> <stdin>

test/cmdlineTests/stdin/stdin

@@ -0,0 +1 @@
+contract C {}