Fix attribute sweep bug and enable more tests on .NET Core (#520)

The introduction of `MethodWithoutParameters()` in this test causes the linker to write invalid IL. When the linker sweeps this method, a bug causes it to keep the attribute on the return type parameter. However, the attribute type itself is removed (causing it to become detached from its declaring type).

https://jenkins.mono-project.com/job/test-linker-pull-request/759/console:
```
09:58:19 1) Failed : Mono.Linker.Tests.TestCases.All.OnlyKeepUsed.UnusedAttributeOnReturnTypeIsRemoved
09:58:19 Invalid IL detected in /tmp/linker_tests/output/library.dll
09:58:19 FAIL: Invalid CustomAttribute row 0 Type field 0x00000022
09:58:19 Error count: 1
09:58:19 at Mono.Linker.Tests.TestCasesRunner.PeVerifier.CheckAssembly (Mono.Linker.Tests.Extensions.NPath assemblyPath) [0x00099] in <0aac952fa2cb4e07af00f27b39783fd3>:0
09:58:19 at Mono.Linker.Tests.TestCasesRunner.PeVerifier.Check (Mono.Linker.Tests.TestCasesRunner.LinkedTestCaseResult linkResult, Mono.Cecil.AssemblyDefinition original) [0x00098] in <0aac952fa2cb4e07af00f27b39783fd3>:0
09:58:19 at Mono.Linker.Tests.TestCasesRunner.ResultChecker.Check (Mono.Linker.Tests.TestCasesRunner.LinkedTestCaseResult linkResult) [0x00095] in <0aac952fa2cb4e07af00f27b39783fd3>:0
09:58:19 at Mono.Linker.Tests.TestCases.All.Run (Mono.Linker.Tests.TestCases.TestCase testCase) [0x00019] in <0aac952fa2cb4e07af00f27b39783fd3>:0
09:58:19 at Mono.Linker.Tests.TestCases.All.AttributesTests (Mono.Linker.Tests.TestCases.TestCase testCase) [0x00001] in <0aac952fa2cb4e07af00f27b39783fd3>:0
```

0 files changed, 0 insertions, 0 deletions