There's no need to check if flushing is needed after every command
is added to the variable holding the script content. It is enough to only
check once per test name. This simplifies the flushing logic, removing
the need for a separate flush_script() command. Previously, we were
not clearing the flushed script contents in all cases, but this is now
rigorously enforced at the one location where flushing is performed.
Also simplify the flushing of the list of test names, since that too doesn't
need a separate command. It is simpler and safer to handle that
directly inline where the one call to flush_tests_buffer() was
previously being made.
Fixes: #26431
Due to add_command() being a macro it introduced excessive and
nonobvious escaping in different parts of the script. Because of
one of such places the resulting script would have an erroneous
${TEST_LIST} if the user data (in test parameters) had a semicolon.
To eliminate this non-obvious escaping, add_command() was converted
to function. Updated the escaping accordingly.
Fixes: #23059
