Add --dev switch

[josephwright]

No description provided.

[muzimuzhi]

Need to also add latex-dev to where all supported checkformat values are listed:

l3build/l3build.dtx

Lines 434 to 436 in 3457c13

	% The format used for tests can be altered by setting \var{checkformat}: the default setting \texttt{latex} means that tests are run using \emph{e.g.}~\texttt{pdflatex}, whereas setting to \texttt{tex} will run tests using \emph{e.g.}~\texttt{pdftex}.
	% (Currently, this should be one of \texttt{latex}, \texttt{tex}, or
	% \texttt{context}.)

[josephwright]

@muzimuzhi Done

[FrankMittelbach]

I think this functionality warrants a ltnews entry (in a different repo I know) explaining how a maintainer of a package using l3build for it can easily test his or her package with the upcoming LaTeX release this way.

[FrankMittelbach]

not sure if it helps or if it makes things worse, but shouldn't there perhaps a terminal output stating --dev not applied if checkformat is not latex?

[josephwright]

I've added one - it only shows at the top level due to the way configs work, but hopefully that's enough?