[wasm] Enable execution in node

[dsmilkov]

Make the WASM backend work in node. Also:

• Enable node tests in CI
• Reduce log outputs when running tests in core.
• Fix Add WASM backend #1497 (comment) (tfjs_backend_wasm.default is not a fuction)
• Remove pre-install script which causes yarn of a third-party project that uses the wasm backend to spin indefinitely.

FEATURE

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This change is 

[nsthorat]

Reviewed 20 of 22 files at r1, 2 of 2 files at r2.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nsthorat)

[dsmilkov]

Reviewable status: complete! 1 of 1 approvals obtained

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tfjs-backend-wasm/tsconfig.json, line 10 at r1 (raw file):

  ],
  "compilerOptions": {
    "esModuleInterop": true,

this fixes #1497 (comment)

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tfjs-core/src/engine_test.ts, line 708 at r1 (raw file):

// WebGL state is initialized, which causes the two backends to step on
// each other and get in a bad state.
describe('Memory allocation outside a test scope', () => {

moved to backend_cpu since it explicitly uses 'cpu' backend, which causes to log the message "Hi there. Looks like you are running...." which causes confusion when running unit tests using the wasm backend.