[ci] Enable native tests for macOS+dotnet test (#806)

Context: #804

Two changes within commit 89a5a22 allow `dotnet test` to be used
to run the net472 mono-required NUnit tests:

 1. The `java-interop` native library is reliably copied to the
    `$(OutputPath)` of referencing projects.  This ensures that e.g.
    `bin/TestDebug/libjava-interop.dylib` exists without extra `make`
    commands or an `msbuild /t:RunTests` invocation.

 2. More importantly, `tests/TestJVM` was updated to use
    `Xamarin.Android.Tools.AndroidSdk.dll` along with
    `JdkInfo.GetKnownSystemJdkInfos()`.

(2) means that the `JI_JVM_PATH` environment variable doesn't *need*
to be set before running unit tests, and is largely responsible for
allowing `dotnet test` to work:

	% make prepare all
	% dotnet test bin/TestDebug/Java.Interop-Tests.dll
	…
	Passed!  - Failed:     0, Passed:   631, Skipped:     1, Total:   632, Duration: 686 ms

Note that these tests are *not* executed under .NET Core/CoreCLR!
It's simply using `dotnet test` as an equivalent to/replacement for
`mono nunit3-console.exe`.

Java.Interop-Tests to be run under .NET Core are built into e.g.
`bin/TestDebug-netcoreapp3.1/Java.Interop-Tests.dll`, and trying to
run *that* test suite crashes and burns:

	% dotnet test bin/TestDebug-netcoreapp3.1/Java.Interop-Tests.dll
	…
	Failed!  - Failed:    16, Passed:   325, Skipped:     1, Total:   342, Duration: 311 ms

See e.g. PR #804 for an attempt to make .NET Core support work.

build-tools/automation/azure-pipelines.yaml

@@ -160,5 +160,7 @@ jobs:
   - template: templates\core-build.yaml
 
   - template: templates\core-tests.yaml
+    parameters:
+      runNativeTests: true
 
   - template: templates\fail-on-issue.yaml

build-tools/automation/templates/core-tests.yaml

@@ -1,5 +1,6 @@
 parameters:
   condition: succeeded()
+  runNativeTests: false
 
 steps:
 - task: DotNetCoreCLI@2
@@ -58,24 +59,26 @@ steps:
     arguments: bin/Test$(Build.Configuration)/Xamarin.SourceWriter-Tests.dll
   continueOnError: true
 
-# Running native Java.Interop tests are not yet supported on .NET Core
-#- task: DotNetCoreCLI@2
-#  displayName: 'Tests: Java.Interop'
-#  inputs:
-#    command: test
-#    arguments: bin/Test$(Build.Configuration)/Java.Interop-Tests.dll
-#  continueOnError: true
+- task: DotNetCoreCLI@2
+  displayName: 'Tests: Java.Interop'
+  condition: eq('${{ parameters.runNativeTests }}', 'true')
+  inputs:
+    command: test
+    arguments: bin/Test$(Build.Configuration)/Java.Interop-Tests.dll
+  continueOnError: true
 
-#- task: DotNetCoreCLI@2
-#  displayName: 'Tests: Java.Interop.Dynamic'
-#  inputs:
-#    command: test
-#    arguments: bin/Test$(Build.Configuration)/Java.Interop.Dynamic-Tests.dll
-#  continueOnError: true
+- task: DotNetCoreCLI@2
+  displayName: 'Tests: Java.Interop.Dynamic'
+  condition: eq('${{ parameters.runNativeTests }}', 'true')
+  inputs:
+    command: test
+    arguments: bin/Test$(Build.Configuration)/Java.Interop.Dynamic-Tests.dll
+  continueOnError: true
 
-#- task: DotNetCoreCLI@2
-#  displayName: 'Tests: Java.Interop.Export'
-#  inputs:
-#    command: test
-#    arguments: bin/Test$(Build.Configuration)/Java.Interop.Export-Tests.dll
-#  continueOnError: true
+- task: DotNetCoreCLI@2
+  displayName: 'Tests: Java.Interop.Export'
+  condition: eq('${{ parameters.runNativeTests }}', 'true')
+  inputs:
+    command: test
+    arguments: bin/Test$(Build.Configuration)/Java.Interop.Export-Tests.dll
+  continueOnError: true