refactor(sdk-trace-base): remove `_registeredSpanProcessors` from BasicTracerProvider

Which problem is this PR solving?

After merging https://github.com/open-telemetry/opentelemetry-js/pull/5152 it is not necessary to have _registeredSpanProcessors private property to modify at runtime. This PR does remove the property and adapts the tests.

Fixes #5136

Short description of the changes

remove _registeredSpanProcessors from BasicTracerProvider
wrap all processors in a MultiSpanProcessor to make sure forceFlush works in all situations.
add a test for it
update tests to not access the removed property

Type of change

Please delete options that are not relevant.

[X] Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

compiled all packages
tested all packages

Checklist:

[X] Followed the style guidelines of this project
[X] Unit tests have been added

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.14%. Comparing base (4a394cc) to head (75b0843). Report is 3 commits behind head on next.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #5177 +/- ## ========================================== - Coverage 93.15% 93.14% -0.02% ========================================== Files 315 315 Lines 8113 8106 -7 Branches 1633 1633 ========================================== - Hits 7558 7550 -8 - Misses 555 556 +1 ``` | [Files with missing lines](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5177?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | Coverage Δ | | |---|---|---| | [...elemetry-sdk-trace-base/src/BasicTracerProvider.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5177?src=pr&el=tree&filepath=packages%2Fopentelemetry-sdk-trace-base%2Fsrc%2FBasicTracerProvider.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-cGFja2FnZXMvb3BlbnRlbGVtZXRyeS1zZGstdHJhY2UtYmFzZS9zcmMvQmFzaWNUcmFjZXJQcm92aWRlci50cw==) | `96.33% <100.00%> (+0.64%)` | :arrow_up: | | [packages/opentelemetry-sdk-trace-base/src/Span.ts](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5177?src=pr&el=tree&filepath=packages%2Fopentelemetry-sdk-trace-base%2Fsrc%2FSpan.ts&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-cGFja2FnZXMvb3BlbnRlbGVtZXRyeS1zZGstdHJhY2UtYmFzZS9zcmMvU3Bhbi50cw==) | `97.61% <ø> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/open-telemetry/opentelemetry-js/pull/5177/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry)

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