Add testing functionality to Amber

[Ph0enixKM]

This PR introduces naming test blocks:

// Unique named test blocks
test "identifier_name" {
  // ...
}

// One nameless block allowed
test {

}

What to do:

• Remove ability to name tests with identifier (leave just the text/string names)
• Make it possible to run a certain test or subset of tests (amber test . "variable" will run all tests that contain substring variable in their names)
• Make it possible to run a specific test (amber test main.ab "Extracts from zip" will run all subset of tests that contain substring Extracts from zip in main.ab file).

This PR also introduces a new standard library std/test with assert(value) and assert_eq(left, right) functions.

Closes #844
Docs amber-lang/amber-docs#133

[Ph0enixKM]

It starts to look pretty cool.

import * from "std/test"

main {
    echo "hello, I'm not being called in tests!"
}

test {
    fail 1
}

test failing {
    assert(false)
}

test this {
    echo "test"
}