Add package config URI into VM test entrypoints

[natebosch]

Towards #2246

This will allow a client of the VM service to evaluate the top level
variable and locate the package config used by the test runner.

Add a public testBootstapContents function in test_compiler.dart
which centralizes the bootstrap templates. Add an argument for the
package config URI and a VmTestType which is either "Vm" or
"Native".

Add the first test case in test_core. Add the pub dependency and
override for package:test, as well as the mono_repo configuration to
run unit tests. Other tests for test_core code is under the test
package but should be moved eventually.

[kenzieschmoll]

This solution works as intended (see flutter/devtools#7941). We should verify this also works from the flutter tools bootstrap generator (CC @christopherfujino)

[kenzieschmoll]

The flutter tools change for this solution is working as intended and awaiting review (flutter/flutter#150440). I recommend we add similar test coverage and code comments in this PR to prevent any future changes to this variable in the generated bootstrap.

[jakemac53]

Now that we have a separate public function, it should be reasonable to add a unit tests that it contains the expected variable.

LGTM once that is in

[natebosch]

We have a bunch of tests for test_core stuff under test, but I'm not going to keep spreading that pattern. I'll add the first set of tests, and configuration to run them, for test_core with the single test case for now. In the future we can migrate some of the tests from test into test_core.

[natebosch]

I noticed that this is only likely to work for dart test. If a VM test is run with flutter test I think the package config we pick up here will be the one for the flutter repo, not the user package.

@kenzieschmoll - when you tested to confirm behavior for the VM tests, did you use dart test only? Or did you try VM tests with flutter test in addition to the platform you modified?

[kenzieschmoll]

This works with flutter test too.