Add configuration option to make assertions thread-safe

All the previous refactoring to make the assertion fast paths
smaller and faster also allows us to implement the fast paths
just with thread-local and atomic variables, without full mutexes.

However, the performance overhead of thread-safe assertions is
still significant for single threaded usage:

|  slowdown |  Debug  | Release |
|-----------|--------:|--------:|
| fast path |   1.04x |   1.43x |
| slow path |   1.16x |   1.22x |

Thus, we don't make the assertions thread-safe by default, and instead
provide a build-time configuration option that the users can set to get
thread-safe assertions.

This commit is functional, but it still needs some follow-up work:
 * We do not need full seq_cst increments for the atomic counters,
   and using weaker ones can be faster.
 * We brute-force updating the reporter-friendly totals from internal
   atomic counters by doing it everywhere. We should properly trace
   where this is needed instead.
 * Message macros (`INFO`, `UNSCOPED_INFO`, `CAPTURE`, etc) are not
   made thread safe in this commit, but they can be made thread safe
   in the future, by building on top of this work.
 * Add more tests, including with thread-sanitizer, and compiled
   examples to the repository. Right now, these changes have been
   compiled with tsan manually, but these tests are not added to CI.

Closes #2948

docs/test-fixtures.md

@@ -4,7 +4,7 @@
 **Contents**<br>
 [Non-Templated test fixtures](#non-templated-test-fixtures)<br>
 [Templated test fixtures](#templated-test-fixtures)<br>
-[Signature-based parameterised test fixtures](#signature-based-parametrised-test-fixtures)<br>
+[Signature-based parameterised test fixtures](#signature-based-parameterised-test-fixtures)<br>
 [Template fixtures with types specified in template type lists](#template-fixtures-with-types-specified-in-template-type-lists)<br>
 
 ## Non-Templated test fixtures