Move test requirements into tests/

This makes the file easier to locate, by placing it next to the stuff
that it will affect.

.github/workflows/ci.yml

@@ -30,7 +30,6 @@ jobs:
             tests:
               # Anything that's touching testable stuff
               - ".github/workflows/ci.yml"
-              - "tools/requirements/tests.txt"
               - "src/**"
               - "tests/**"
         if: github.event_name == 'pull_request'

tox.ini

@@ -23,10 +23,10 @@ setenv =
     # This is required in order to get UTF-8 output inside of the subprocesses
     # that our tests use.
     LC_CTYPE = en_US.UTF-8
-deps = -r{toxinidir}/tools/requirements/tests.txt
+deps = -r{toxinidir}/tests/requirements.txt
 commands_pre =
     python -c 'import shutil, sys; shutil.rmtree(sys.argv[1], ignore_errors=True)' {toxinidir}/tests/data/common_wheels
-    {[helpers]pip} wheel -w {toxinidir}/tests/data/common_wheels -r {toxinidir}/tools/requirements/tests-common_wheels.txt
+    {[helpers]pip} wheel -w {toxinidir}/tests/data/common_wheels -r {toxinidir}/tests/requirements-common_wheels.txt
 commands = pytest []
 install_command = {[helpers]pip} install {opts} {packages}
 list_dependencies_command = {[helpers]pip} freeze --all