[test] Update & reenable stdlib ABI stability tests

[lorentey]

Put the list of expected changes directly in the test file to make it simpler to update. Encourage Build Wranglers to extend the list instead of xfailing it -- the latter masks not just existing ABI breaks but future ones too.

[lorentey]

@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - a8c785b

[lorentey]

@swift-ci test linux platform

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - a8c785b

[lorentey]

Linux platform tests failed in an unrelated problem with SwiftPM:

13:42:05 Test Case 'CFamilyTargetTestCase.testCUsingCAndSwiftDep' started at 2020-07-20 20:41:42.906
13:42:05     Building Debug
13:42:05     Building Release
13:42:05 
/home/buildnode/jenkins/workspace/swift-PR-Linux/branch-master/swiftpm/Tests/FunctionalTests/CFamilyTargetTests.swift:48: error: CFamilyTargetTestCase.testCUsingCAndSwiftDep : failed - `swift build -c Release' failed:
13:42:05 
13:42:05 executionFailure(error: terminated(1): /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-master/buildbot_linux/swiftpm-linux-x86_64/x86_64-unknown-linux-gnu/release/swift-build --package-path /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-master/tmp/DependencyResolution_External_CUsingCDep.aFui6p/Bar --configuration release output:
13:42:05     [1/3] Compiling Foo Foo.c
13:42:05     [2/3] Compiling SeaLover Sea.c
13:42:05     [3/4] Compiling SwiftExec main.swift
13:42:05     <unknown>:0: warning: missing submodule 'Foo'
13:42:05 
    /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-master/tmp/DependencyResolution_External_CUsingCDep.aFui6p/Bar/Sources/SwiftExec/main.swift:1:8: error: no such module 'Foo'
13:42:05     import Foo
13:42:05            ^
13:42:05     , output: "[1/3] Compiling Foo Foo.c\n[2/3] Compiling SeaLover Sea.c\n[3/4] Compiling SwiftExec main.swift\n<unknown>:0: warning: missing submodule \'Foo\'\n/home/buildnode/jenkins/workspace/swift-PR-Linux/branch-master/tmp/DependencyResolution_External_CUsingCDep.aFui6p/Bar/Sources/SwiftExec/main.swift:1:8: error: no such module \'Foo\'\nimport Foo\n       ^\n", stderr: "")

[lorentey]

Let's try that again, shall we

@swift-ci test linux platform