Configure dependabot to bump test/Directory.Build.props

[pellared]

Fixes #356

Changes proposed in this pull request:

• Configure dependabot to bump references defined in test/Directory.Build.props.

Kudos to @lachmatt for his initial work that led to this solution.

Testing

I changed the default branch to dependabot-for-propos on my fork and depdabot created:

• Bump Moq from 4.16.1 to 4.18.1 in /test pellared/opentelemetry-dotnet-instrumentation#130
• Bump xunit.runner.visualstudio from 2.4.3 to 2.4.5 in /test pellared/opentelemetry-dotnet-instrumentation#129
• Bump FluentAssertions from 6.4.0 to 6.7.0 in /test pellared/opentelemetry-dotnet-instrumentation#128
• Bump Microsoft.NET.Test.Sdk from 17.0.0 to 17.2.0 in /test pellared/opentelemetry-dotnet-instrumentation#127

Output from dependabot logs:

updater | +---------+---------------------------------------------------+
updater | |             Changes to Dependabot Pull Requests             |
updater | +---------+---------------------------------------------------+
updater | | created | Microsoft.NET.Test.Sdk ( from 17.0.0 to 17.2.0 )  |
updater | | created | FluentAssertions ( from 6.4.0 to 6.7.0 )          |
updater | | created | xunit.runner.visualstudio ( from 2.4.3 to 2.4.5 ) |
updater | | created | Moq ( from 4.16.1 to 4.18.1 )                     |
updater | +---------+---------------------------------------------------+