TST Improve xbuildenv and emscripten mocks

[ryanking13]

Description

The goal of this PR is to reduce the dependency on pyodide for testing pyodide-build. The main changes are:

1. Add a new fixture: mock_emscripten for mocking emscripten compiler toolchain. This fixture will create a dummy emcc (+ other toolchains) to the PATH. When running tests, all emcc calls will be intercepted by this command.

2. Improve xbuildenv fixture. Previously, this fixture actually created an xbuildenv from the Pyodide artifacts. So, we had to have Pyodide already built before running the tests. In this PR, this fixture now uses a pre-built minimum xbuildenv.

As a result, now all tests in pyodide-build run without building Pyodide. The only exception is test_xbuildenv_create which test pyodide xbuildenv create command. In a follow-up, I am planning to migrate the provide venv tests, which are in test_cmdline_runner.py.

Checklists

• Add / update tests

[hoodmane]

Thanks @ryanking13!