Use configurable binaries directory in the tests

Locate curl-impersonate and libcurl-impersonate in a directory which is
configurable from the command line instead of looking for them in the
current directory. '--install-dir' is passed to pytest, where a 'bin'
and 'lib' directories are expected with (lib)curl-impersonate.

Rename the Actions file for the Docker build to allow the creation of
addition Action for native builds.

tests/conftest.py

@@ -0,0 +1,4 @@
+def pytest_addoption(parser):
+    # Where to find curl-impersonate's binaries
+    parser.addoption("--install-dir", action="store", default="/usr/local")
+    parser.addoption("--capture-interface", action="store", default="eth0")