Have OpenSSL::Test handle perl scripts like any program

Since we're building some of our perl scripts and the result might not
end up in apps/ (*), we may need to treat them like the compile
programs we use for testing.

This introduces perlapp() and perltest(), which behave like app() and
test(), but will add the perl executable in the command line.

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(*) For example, with a mk1mf build, the result will end up in $(BIN_D)

Reviewed-by: Matt Caswell <matt@openssl.org>