Only show test trace execution when the CLAR_TRACE_TESTS environment
variable is set.  This reduces the noise during tracing.

Tracing is meant to be extremely low-impact when not enabled.  We
currently ship no tracing calls in libgit2, but if / when we do, the
tracing infrastructure is created to skip tracing as quickly as
possible.  It should compile to a simple test when tracing is off.

Thus, there's on reason to not enable it by default.

CI Build Updates

Provide usage hints to valgrind.  We trust the data coming back from
OpenSSL to have been properly initialized.  (And if it has not, it's an
OpenSSL bug, not a libgit2 bug.)

We previously took the `VALGRIND` option to CMake as a hint to disable
mmap.  Remove that; it's broken.  Now use it to pass on the `VALGRIND`
definition so that sources can provide valgrind hints.

valgrind will warn that OpenSSL will use undefined data in connect/read
when talking to certain other TLS stacks.  Thankfully, this only seems
to occur when gcc is the compiler, so hopefully valgrind is just
misunderstanding an optimization.  Regardless, suppress this warning.

We currently talk to Azure Repos for executing an online test
(online::clone::path_whitespace).  Add a simpler test to talk to Azure
Repos to make it obvious that strange test failures are not likely the
whitespace in the path, but actually a function of talking to Azure
Repos itself.

Our docker builds are getting expensive, let's cache some of this.