Bug: let() is invoked before beforeEach

[GuyPaddock]

In RSpec, let blocks are evaluated and cached lazily in each spec only when their value is first needed within a given test. But, in Spectrum, it appears that the let blocks all fire off and get cached at the start of the test. In fact, they all actually seem to get invoked before any beforeEach blocks. This means that let blocks that depend on any state that the beforeEach blocks manipulate will not be correct.

Here is a test that demonstrates the issue:

package com.rosieapp.test;

import static com.greghaskins.spectrum.dsl.specification.Specification.beforeEach;
import static com.greghaskins.spectrum.dsl.specification.Specification.context;
import static com.greghaskins.spectrum.dsl.specification.Specification.it;
import static com.greghaskins.spectrum.dsl.specification.Specification.let;
import static org.assertj.core.api.Assertions.assertThat;

import com.greghaskins.spectrum.Spectrum;
import com.greghaskins.spectrum.Variable;
import java.util.function.Supplier;
import org.junit.runner.RunWith;

@RunWith(Spectrum.class)
public class BadTest {
  {
    final Variable<String> theVariable = new Variable<>();
    final Supplier<String> result      = let(() -> theVariable.get());

    context("when `theVariable` is `x`", () -> {
      beforeEach(() -> {
        theVariable.set("x");
      });

      it("stores `x` in `result`", () -> {
        // Fails: `result` is `null` because it was fetched & cached before the `beforeEach`
        assertThat(result.get()).isEqualTo("x");
      });
    });

    context("when `theVariable` is `y`", () -> {
      beforeEach(() -> {
        theVariable.set("y");
      });

      it("stores `y` in `result`", () -> {
        // Fails: `result` is `x` because it was fetched & cached before the `beforeEach`
        assertThat(result.get()).isEqualTo("y");
      });
    });
  }
}

To work around this limitation, I'm having to use beforeAll() at the start of each context. That works (and is a bit more efficient since we only have to reset the variable once per context), but introduces the possibility that each of the tests within a context are sharing more state than desired.

[GuyPaddock]

This sounds related to #127, though I'm using specification syntax rather than Gherkin.