add a new function `Assert` to do some assertions in test

[lcwangchao]

Enhancement

Usually, we have some constraints in code, but it is not applicable to check it every time in the production environment. So we propose a new function Assert to do some assertions. The Assert function only works in test environment, and in a release build, it will return directly. It's implement is like this:

func Assert(condition any, msgAndArgs ...any) {
    if intest.Intest { // intest.Intest is true only in test env
        doAssert(condition, msgAndArgs...)
    }
}

We can use this function like below:

var foo *Foo
foo = ...

 // assert a bool is true
intest.Assert(foo != nil);

// or directly assert a pointer is not nil
intest.Assert(foo)

// assert the function's return value is true
intest.AssertFunc(func() bool) {
    return foo != nil
}

// with some custom messages
intest.Assert(foo != nil, "foo should not be nil")
intest.Assert(a == 0, "a should be zeror, but current is %d", a)

if you have some code that is sensitive in performance and don't want extra cost in stack jump, you can check if intest.Intest is true before assert.

if intest.Test {
    assert.Assert(foo != nil)
}

However, I wonder if it is necessary because go will auto inline simple functions in most cases.