test: don't try to use Xcode SDK for build requirement

[Rylan12]

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Previously, running brew test failed if the formula had an Xcode build requirement but the user did not have Xcode installed. brew would see the Xcode requirement and default to the Xcode SDK. However, for formula that only need Xcode to build, this check is unnecessary when simply testing the formula. I ran into this in Homebrew/homebrew-core#67043 with deno:

$ brew test deno
==> Testing deno
Error: deno: failed
An exception occurred within a child process:
  NoMethodError: undefined method `path' for nil:NilClass
Did you mean?  paths
/usr/local/Homebrew/Library/Homebrew/extend/os/mac/extend/ENV/std.rb:88:in `macosxsdk'
/usr/local/Homebrew/Library/Homebrew/extend/os/mac/extend/ENV/std.rb:45:in `setup_build_environment'
/usr/local/Homebrew/Library/Homebrew/test.rb:39:in `<main>'

I don't have code installed, so Xcode.sdk.path failed (Xcode.sdk is nil).

My solution is to, when testing, pass a flag to the macosxsdk method to determine whether to check only runtime requirements. When this is false (the default), any Xcode requirement will require the use of the Xcode SDK. When set to true (only in brew test), build-only Xcode requirements will not necessarily require the Xcode SDK.

I'm marking this PR as critical because I believe it fixes a bug. However, I'm not familiar at all with the files that I'm changing so it's entirely possible that this change will have unintended consequences. I will not merge until I have approvals from other maintainers who are more familiar with this code.

[BrewTestBot]

Review period ended.

[carlocab]

Tested this locally with the existing deno bottle:

❯ brew test deno
==> Testing deno
==> /usr/local/Cellar/deno/1.6.0/bin/deno run hello.ts
Check file:///private/tmp/deno-test-20201218-47524-x10fau/hello.ts
==> /usr/local/Cellar/deno/1.6.0/bin/deno run --allow-read=/private/tmp/deno-test-20201218-47524-x10fau https://deno.land/std@0.50.
Download https://deno.land/std@0.50.0/examples/cat.ts
Check https://deno.land/std@0.50.0/examples/cat.ts

Works perfectly fine.

[Rylan12]

Updated to simplify/clarify the logic and switch to using keyword arguments instead of **options

[MikeMcQuaid]

Thanks @Rylan12!