Include a self-signed certificate for test.libgit2.org:1443 that we can
use to verify that GIT_OPT_SET_SSL_CERT_LOCATIONS works.

(And move the ci scripts into the `ci` directory.)

Since we have migrated to Azure Pipelines, we have deprecated and
subsequentally removed all infrastructure for AppVeyor and
Travis. Thus it doesn't make a lot of sense to have the split
between "ci/" and "azure-pipelines/" directories anymoer, as
"azure-pipelines/" is essentially our only CI.

Move all CI scripts into the "azure-pipelines/" directory to have
everything centrally located and to remove clutter in the
top-level directory.

Right now, we have an awful hack in our test CI setup that extracts the
test command from CTest's output and then prepends the leak checker.
This is dependent on non-machine-parseable output from CMake and also
breaks on various ocassions, like for example when we have spaces in the
current path or when the path contains backslashes. Both conditions may
easily be triggered on Win32 systems, and in fact they do break our
Azure Pipelines builds.

Remove the awful hack in favour of a new CMake build option
"USE_LEAK_CHECKER". If specifying e.g. "-DUSE_LEAK_CHECKER=valgrind",
then we will set up all tests to be run under valgrind. Like this, we
can again simply execute ctest without needing to rely on evil sourcery.

Instead of having to find the fuzzer executables in our Azure test
scripts, provide test targets for each of our fuzzers that will
run them with the correct paths.

Our online tests are occasionally flaky since they hit real network
endpoints.  Re-run them up to 5 times if they fail, to allow us to
avoid having to fail the whole build.

Update the proxy so that we can enable a quiet mode.