Add fixtures to requirements

The commit 887c445 made the nova.utils
module dependent on the fixtures library but the change missed updating
requirements and the fixtures library is not installed automatically.

This change migrates the fixtures library from test-requirements.txt to
requirements.txt so that the library is installed without test codes.

Closes-Bug: #1958075
Change-Id: I712f88fc1b6053fe6d1f13e708f3bd8874452a8f

Author: kajinamit

requirements.txt

@@ -68,3 +68,4 @@ futurist>=1.8.0 # Apache-2.0
 openstacksdk>=0.35.0 # Apache-2.0
 dataclasses>=0.7;python_version=='3.6'  # Apache 2.0 License
 PyYAML>=5.1 # MIT
+fixtures>=3.0.0 # Apache-2.0/BSD

test-requirements.txt

@@ -7,7 +7,6 @@ mypy>=0.761 # MIT
 types-paramiko>=0.1.3 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.2.1 # MIT
-fixtures>=3.0.0 # Apache-2.0/BSD
 mock>=3.0.0 # BSD
 psycopg2-binary>=2.8 # LGPL/ZPL
 PyMySQL>=0.8.0 # MIT License