Elements. Remove unit-like tests from test/src/dart/element/element_test.dart

We will rely on other tests that build the elements from code,
not these that build synthetic elements.

Change-Id: I90a1597880e6e1fa217d3a18f97561ec188b13d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/416040
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov