[flang][test] Adjust Driver/pass-plugin-not-found.f90 error message

The `Driver/pass-plugin-not-found.f90` test `FAIL`s on Solaris since part
of the error message differs from what the test expects: Solaris has

  error: unable to load plugin 'X.Y': 'Could not load library 'X.Y':
ld.so.1: flang-new: X.Y: open failed: No such file or directory'

while the test expects

  error: unable to load plugin 'X.Y': 'Could not load library 'X.Y': X.Y:
cannot open shared object file: No such file or directory'

The last part is emitted by the system `dlerror` and is thus unportable.
Hopefully the error text itself is the same everywhere.

The following patch adjusts the expected message accordingly.

Tested on `x86_64-pc-linux-gnu` and `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D138663