We centralize the creation paths for test indexes, fields,
etcetera so they all have a common path, all using standard
test holders. There's still two versions, one for test.* functions
and one for internal. They do share a TestHolderConfig though.
Large hunks of the related APIs are simplified/streamlined.
* Fragments are always created with a Field and don't need
a workaround in case they don't have it.
* Creation of test fragments, etc., use optional FieldOptions
but don't specify names because they're all using new holders
for each thing created anyway. This dramatically reduces
the complexity of the calls.
* test fragments are created inside test views which are created
inside test fields, etcetera, so everything is using the same
logic; test views aren't bypassing the other layers, they're
creating themselves normally within a field.
* Quite a few things now use the standard runtime/production
logic instead of being custom workarounds; for instance, instead
of `mustOpenMutexFragment` creating a fragment and then creating
a mutex vector for it, we just create a mutex-typed field and
have the normal runtime code do this.
* Similarly, we now use the same field creation logic that production
does, instead of having our own test-only thing that validates
field names directly, so our test that we're validating field names
is actually testing the runtime code. Yay.
* fragSpec goes away. it was a replacement for fragProxy which existed
to solve memory allocation problems but replaced them with interface
overhead problems. Now we just have pointers to things and maintain
valid data structures.
* Many panics are now Fatal or Fatalf calls.
* Some specific bugs fixed, like a cluster which was requested and
then had its first node directly overwritten, which isn't valid with
shared clusters.
* Drop the temp-dir test flag and TempDir variable, we can just use
$TMPDIR.
* Drop a benchmark of "write file to disk" that was purely a benchmark
of file write speed, not a benchmark of rendering the data that needs
to be written.
* Drop the unused "flags" parameter to fragment creation, which was
only used back when we changed the BSI format.
* Use holder.Txf() rather than index.Txf(). The TxFactory has to be
holder-level anyway, referring to it via the index is misleading.
* Test holders automatically close themselves and delete themselves,
we remove various other things that thought they were responsible
for deleting themselves.
