selftests: forwarding: README: Document customization

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Customization

=============

The forwarding selftests framework uses a number of variables that

influence its behavior and tools it invokes, and how it invokes them, in

various ways. A number of these variables can be overridden. The way these

overridable variables are specified is typically one of the following two

syntaxes:

	: "${VARIABLE:=default_value}"

	VARIABLE=${VARIABLE:=default_value}

Any of these variables can be overridden. Notably net/forwarding/lib.sh and

net/lib.sh contain a number of overridable variables.

One way of overriding these variables is through the environment:

	PAUSE_ON_FAIL=yes ./some_test.sh

The variable NETIFS is special. Since it is an array variable, there is no

way to pass it through the environment. Its value can instead be given as

consecutive arguments to the selftest:

	./some_test.sh swp{1..8}

A way to customize variables in a persistent fashion is to create a file

named forwarding.config in this directory. lib.sh sources the file if

present, so it can contain any shell code. Typically it will contain

assignments of variables whose value should be overridden.

forwarding.config.sample is available in the directory as an example of

how forwarding.config might look.