Refactor ClientLoader.clientTests

[osipxd]

Subsystem
Test Infrastructure

Motivation
KTOR-7967 Make client tests independent

Solution
There were several issues with the clientTests function:

• On JS/WasmJS platform Mocha timeout was applied, so it was impossible to set a bigger timeout for tests individually
• The configured timeout was applied to the whole test, including different clients and retries. So after the timeout error, there was no sense in retrying,

Refactored client tests to apply timeout to each attempt individually.
The main difficulty of this refactor is JS/WasmJS support. runTest returns TestResult, which is mapped to Promise<Unit> on JS platform, so we can't just retry the test in a loop. For JS we should use Promise.then { ... } and Promise.catch { ... }.

Depends on:

• Cherry-pick common ClientLoader implementation from 3.1.0-eap #4550

[bjhham]

Nice 🥇