[MAINT] Fixed example buildbreaks. Added examples for message mode transmission

[ethouris]

Changes:

1. Fixed build breaks in the C examples. Broken in [examples] add wait 1 sec until receiver completes. #2572.
2. Added building examples and testing (devel apps) to one of the CI builds.
3. Added example files for simple testing the message mode.

The message mode testing can be done the following way:

1. recvmsg hosts a listener and receives messages, then displays them one after another on the screen, or writes them to subsequent files.
2. sendmsg reads messages each one with a single line. Optionally the line can start with + to additionally define a TTL for it. Optionally the input specification can start with + to define the input being a list of lines with filenames. Every line can also contain +N defining a TTL for this message, and the content is read from the given file. First 4 bytes of the message is an identifier, in case of direct line contents it's specified additionally, and for filenames they are ordinal numbers starting from 1.

[ethouris]

NOTE: these files for testing the message mode are necessary for a test-gate for the fix for the change in the code around message dropping. They are necessary to test how the message dropping is handled in the SRT code.