Update CATCH to version 2.5.0

[tautschnig]

Downloaded via
https://github.com/catchorg/Catch2/releases/download/v2.5.0/catch.hpp

Version 2 also addresses the only local change we ever had, which was adding
"const &" to the exception type.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 4f780a3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98209867
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 02fe307).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98331058
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[tautschnig]

@allredj @peterschrammel @thk123 This seems to continue to trip up TG. I'm not actually sure why, though. Could you please take a look?

[allredj]

@tautschnig Looks like this one also needs a manual bump. Will look at this tomorrow.

[allredj]

TG update: https://github.com/diffblue/test-gen/pull/2861

[allredj]

@tautschnig
TG bump passing. Can be merged.

[tautschnig]

Thanks a lot @allredj!!