Support setting DENO_TLS_CA_STORE for testing

[jonasb]

Is your feature request related to a problem? Please describe.

When running tests I need to use the system TLS CA store wo use the system store, e.g. env DENO_TLS_CA_STORE=system deno test. I haven't found a good way to set that environment variable when running tests through VS Code.

Describe the solution you'd like

Either a setting specifically for DENO_TLS_CA_STORE for running tests, or a generic way of setting environment variables for tests.

[ahuigo]

I suggest to support this deno.codeLens.testEnvs option