fix($compile): set attribute value even if `ngAttr*` contains no interpolation

Previoulsy, when the value of an `ngAttrXyz` attribute did not contain any interpolation, then the
`xyz` attribute was never set.

BTW, this commit adds a negligible overhead (since we have to set up a one-time watcher for
example), but it is justifiable for someone that is using `ngAttrXyz` (instead of `xyz` directly).

(There is also some irrelevant refactoring to remove unnecessary dependencies from tests.)

Fixes #15133

Closes #15149