Support multiple conditions in [ConditionalFact] and [ConditionalTheory]

[roncain]

Modifies [ConditionalFact] and [ConditionalTheory] attributes to accept
a variable number of condition member names.

Modifies ConditionalFactDiscoverer and ConditionalTheoryDiscoverer to
evaluate all of the conditions in the attributes. Conditions are
logically "and'ed", so any condition returning false marks the test
to be skipped.

Existing tests using [ConditionalFact] or [ConditionalTheory] do not
need to be modified. The only behavioral change is the phrasing of
the skip message which now begins "Conditions not met:..."

Fixes #610

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[roncain]

Reviewers: this was tested by manually creating the following tests:

   [ConditionalFact()]
    public static void Test_No_Conditions()
    {
        Assert.True(true);
    }

    [ConditionalFact(nameof(Running_Elevated))]
    public static void Test_One_False_Conditions()
    {
        Assert.True(false);
    }

    [ConditionalFact(nameof(Certificates_Installed))]
    public static void Test_One_True_Condition()
    {
        Assert.True(true);
    }

    [ConditionalFact(nameof(Running_Elevated), nameof(Domain_Joined), nameof(Certificates_Installed))]
    public static void Test_Multiple_False_Conditions()
    {
        Assert.True(false);
    }

    [ConditionalFact(nameof(Built_On_Friday), nameof(Certificates_Installed))]
    public static void Test_Multiple_True_Conditions()
    {
        Assert.True(true);
    }

    [ConditionalTheory()]
    [InlineData("testdata")]
    public static void Test_No_Theory_Conditions(string theoryInput)
    {
        Assert.True(true);
    }

    [ConditionalTheory(nameof(Running_Elevated))]
    [InlineData("testdata")]
    public static void Test_One_False_Theory_Conditions(string theoryInput)
    {
        Assert.True(false);
    }

    [ConditionalTheory(nameof(Certificates_Installed))]
    [InlineData("testdata")]
    public static void Test_One_True_Theory_Condition(string theoryInput)
    {
        Assert.True(true);
    }

    [ConditionalTheory(nameof(Running_Elevated), nameof(Domain_Joined), nameof(Certificates_Installed))]
    [InlineData("testdata")]
    public static void Test_Multiple_False_Theory_Conditions(string theoryInput)
    {
        Assert.True(false);
    }

    [ConditionalTheory(nameof(Built_On_Friday), nameof(Certificates_Installed))]
    [InlineData("testdata")]
    public static void Test_Multiple_True_Theory_Conditions(string theoryInput)
    {
        Assert.True(true);
    }

The result of running these tests looks like this:

Discovering: System.ServiceModel.Primitives.Tests
Discovered:  System.ServiceModel.Primitives.Tests
Starting:    System.ServiceModel.Primitives.Tests
   ChannelFactoryTest.Test_One_False_Conditions [SKIP]
      Condition(s) not met: "Running_Elevated"
   ChannelFactoryTest.Test_Multiple_False_Theory_Conditions [SKIP]
      Condition(s) not met: "Running_Elevated", "Domain_Joined"
   ChannelFactoryTest.Test_One_False_Theory_Conditions [SKIP]
      Condition(s) not met: "Running_Elevated"
   ChannelFactoryTest.Test_Multiple_False_Conditions [SKIP]
      Condition(s) not met: "Running_Elevated", "Domain_Joined"
Finished:    System.ServiceModel.Primitives.Tests

=== TEST EXECUTION SUMMARY ===
   System.ServiceModel.Primitives.Tests  Total: 89, Errors: 0, Failed: 0, Skipped: 4, Time: 40.544s

/cc @stephentoub @iamjasonp @Priya91

[stephentoub]

LGTM. Thanks.

[roncain]

Thanks @stephentoub . Is it okay for me to merge or would you like others eyes on this? I am considering your option 3 (special exception throw for custom skip messages) for a future enhancement to these discoverers, but not as part of this PR.

[stephentoub]

@Priya91, could you take a look as well?

[bartonjs]

I'm all done being nitpicky. Thanks, Ron :).

[roncain]

I'm waiting for feedback from @Priya91 or @iamjasonp and will merge when I hear back. Alternatively, anyone else can feel free to merge at any time. Feedback from @bartonjs was incorporated into the 2nd commit.

[iamjasonp]

Nice 😄 LGTM

[Priya91]

I was OOF yesterday, and saw this only now. LGTM! :)