Tests: Fix xcode version detection for Xcode 13 on ARM

Running `xcode -version` on an ARM box prints content to stderr:

    objc[...]: Class ... is implemented in both...
    One of the two will be used. Which one is undefined.

Capture and parse only stdout.  Otherwise we might configure test
behavior for the wrong version of Xcode.
58 jobs for !6248 with test-xcode-13 in 15 minutes and 7 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build
manual #5888253
linux docker build cmake manual
build:centos6-x86_64
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linux docker build cmake
build:debian10-iwyu

00:06:42

manual #5888254
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
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linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:03:03

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cmake linux-3.17 linux docker build
build:fedora33-tidy

00:15:07

manual #5888256
nonconcurrent xcode-12.5 shell macos cmake manual
build:macos-x86_64-makefiles
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nonconcurrent xcode-12.5 shell macos cmake manual
build:macos-x86_64-ninja
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nonconcurrent msvc-19.29-16.10 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja
 
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x11 linux-3.17 linux docker cmake
test:fedora33-ninja
created #5888260
nonconcurrent xcode-12.5 shell macos cmake
test:macos-x86_64-makefiles
created #5888259
nonconcurrent xcode-12.5 shell macos cmake
test:macos-x86_64-ninja
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nonconcurrent msvc-19.29-16.10 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja
 
  Test Ext
created #5888264
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5888262
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5888263
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5888265
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5888266
concurrent xcode-12.5 shell macos cmake
test:macos-x86_64-xcode
created #5888267
concurrent msvc-19.29-16.10 vs2019 shell windows cmake
test:windows-vs2019-x64
 
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