Try to run android tests on simulator

[nirinchev]

Description

Rather than wait for a proper device farm action, this just executes android tests on the emulator on macOS. It's not ideal, but will allow us to nuke the jenkins build which has been unstable for a while.

In addition, this PR does the following:

• Changes how arguments are getting passed to tests. Previously we would expect the argument and the value to be passed separately - i.e. --foo bar, whereas now we're expecting them to be passed with = to be consistent with the NUnit format.
• The test result argument was --resultpath and --result on different platforms. It's now --result everywhere.
• Bumps NUnit version to 3.13.2 (latest). This was required because older NUnit versions were shipping debug symbols in a now-deprecated format.
• Moved SplitArguments to TestHelpers from UWPTestHelpers because it's now used on Android as well.
• Android's MainActivity now expects arguments to be passed as a single string and splits them. It also handles correctly the BaaS arguments now.
• Bump Android tests target version to 29.
• Android instrumentation tests are now going to read arguments from *external storage*/RealmTests/testargs.txt if it exists.
• Streamlined the Xamarin.macOS test runner a little.

Fixes #2288

TODO

• Changelog entry