Avoid assuming ./mlr is the mlr to test

[skitt]

In test cases, support a ${MLR} placeholder for the mlr binary under
test, instead of assuming it's ./mlr (or its equivalent on platforms
not using forward slashes).

This ensures that wrapped mlr invocations all use the same binary,
either the default or the binary specified by the user, avoiding
surprises when ./mlr doesn't exist or is a different version.

Signed-off-by: Stephen Kitt steve@sk2.org

[skitt]

This could be used throughout the test cases to avoid the special handling of "mlr " at the beginning of the command, but I wanted to keep this simple for now and provide a straightforward fix for the ./mlr issue.

[johnkerl]

Thanks @skitt !!