Improve Python tests (aws-observability#908)

* Create requirements file for dependencies used in tests
* Update tox file to use dependencies file with stable dependencies version

Author: rapphil

python/src/otel/tests/requirements.txt

@@ -0,0 +1,3 @@
+# Dependencies used in tests only
+opentelemetry-test-utils==0.40b0
+opentelemetry-instrumentation-aws-lambda==0.40b0

python/src/tox.ini

@@ -14,18 +14,12 @@ passenv = TOXENV
 
 setenv =
     OTEL_PYTHON_TRACER_PROVIDER=sdk_tracer_provider
-    ; override CORE_REPO_SHA via env variable when testing other branches/commits than main
-    ; i.e: CORE_REPO_SHA=<YOUR_COMMIT_SHA> tox -e test-instrumentation-aws-lambda
-    CORE_REPO_SHA={env:CORE_REPO_SHA:main}
-    CORE_REPO="git+https://github.com/open-telemetry/opentelemetry-python.git@{env:CORE_REPO_SHA}"
 
 changedir =
     test-instrumentation-aws-lambda: {toxinidir}/otel/tests
 
 commands_pre =
-    test: pip install "opentelemetry-test-utils[test] @ {env:CORE_REPO}\#egg=opentelemetry-test-utils&subdirectory=tests/opentelemetry-test-utils"
-
-    aws-lambda: pip install opentelemetry-instrumentation-aws-lambda
+    test: pip install -r requirements.txt
 
 deps =
     test: pytest