Update R plotting metadata check

[chinandrew]

Fixes #270

Summary of changes

• Update the metadata check in the plotting function to look specifically at mean and stdev values
• Add test that the warning is raised if one of the values is missing and not raised if both are present.

[capnrefsmmat]

Thanks; a couple suggested improvements

[capnrefsmmat]

Having a warning class means we don't have to match the message:

Suggested change

	"Metadata for signal mean and standard deviation not",
	"available; defaulting to observed mean and standard",
	"deviation to set plot range.")
	class = "covidcast_plot_meta_not_found")

[chinandrew]

So I tried this, but couldn't get it to work since this passes (as expected)

  expect_warning(plot(fake_data), class="covidcast_plot_meta_not_found")

but this also passes

  expect_warning(plot(fake_data), class="this_should_fail")

[capnrefsmmat]

wat

sigh:

> expect_warning(warn("ducks", class="duck_warning"), class="boing")
> expect_error(abort("ducks", class="duck_error"), class="boing")
Error: `abort("ducks", class = "duck_error")` threw an error with unexpected class.
Expected class: boing
Actual class:   duck_error/rlang_error/error/condition
Message:        ducks

So I guess the documentation is a lie for warnings. We might be able to fix this by opting in to testthat's third edition, since that unifies the handling for all three. Worth trying?

[chinandrew]

Seems to work

> library(testthat)
> local_edition(3)
> expect_warning(warn("ducks", class="duck_warning"), class="boing")
Error: `warn("ducks", class = "duck_warning")` did not throw the expected warning.
In addition: Warning message:
ducks 

Updated code to use 3rd ed