Add tests for bem-xjst4 compatibility flags

[vkz]

• assertHasBlock assert that every template has sub-predicate matching block
• assertNoThisElem assert there're no predicates matching this.elem bem/bem-xjst#89
• applySetsMode assert that every apply sets the mode as its first argument bem/bem-xjst#91
• applyCheckFields warn when apply assigns block, elem, mods, elemMods bem/bem-xjst#73
• returnFromDef ensure there's always return in def mode. Insert return '' when there is none
• 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).