Remove "content" locator from acceptance tests

Remove "content" locator from acceptance tests

The "content" locator uses the "named" Mink selector and the "content"
Mink locator to find the element. The "named" Mink first tries to find
the elements whose content match exactly the given content but, if none
is found, then it tries to find elements that just contain the given
content.

This behaviour can lead to hard to track issues. Finding the exact match
and, if not found, finding the partial match is done in quick
succession. In most cases, when looking for an exact match the element
is already there, it is returned, and everything works as expected. Or
it may not be there, but then it is not there either when finding the
partial match, so no element is returned, and everything works as
expected (that is, the actor tries to find again the element after some
time).

However, it can also happen that when looking for an exact match there
is no element yet, but it appears after trying to find the exact match
but before trying to find the partial match. In that situation the
desired element would be returned along with its ancestors. However, as
only the first found element is taken into account and the ancestors
would appear first the find action would be successful, but the returned
element would not be the expected one. This is highly unlikely, yet
possible, and can cause sporadic failures in acceptance tests that,
apparently, work as expected.

Using a "named_exact" Mink selector instead of the "named" Mink selector
does not provide the desired behaviour in most cases either. As it finds
any element whose content matches exactly the given content, looking for
"Hello world" in "<div><p><a>Hello world</a></p></div>" would match the
"div", "p" and "a" elements; in that situation the "div" element would
be the one returned, when typically the "a" element would be the
expected one.

As it is error prone and easily replaceable by more robust locators the
"content" locator was removed from the predefined ones (although it can
still be used if needed through the "customSelector" method in the
builder object).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>