Add tests for bem-xjst4 compatibility flags

[vkz]

• [x] assertHasBlock assert that every template has sub-predicate matching block
• [x] assertNoThisElem assert there're no predicates matching this.elem https://github.com/bem/bem-xjst/issues/89
• [ ] applySetsMode assert that every apply sets the mode as its first argument https://github.com/bem/bem-xjst/issues/91
• [ ] applyCheckFields warn when apply assigns block, elem, mods, elemMods https://github.com/bem/bem-xjst/issues/73
• [x] returnFromDef ensure there's always return in def mode. Insert return '' when there is none
• [x] assertNoBuf assert this._buf is never used in templates

[vkz]

applySetsMode and applyCheckFields are just checks that only warn, never assert. Both work but I'm not adding them to test-suite explicitly (for now).