Consolidate environment variable lists

We want to make sure that all of our test environments are clean from
possible outside modification. This consolidates the list of used
environment variables in Rust-based and Python-based integration tests.

Note that there is also a similar list in `src/bin/bat/main.rs` which
is even more exhaustive (for bug report collection). However, some
of these variables can not possibly have an effect on test environments.

tests/syntax-tests/create_highlighted_versions.py

@@ -47,12 +47,16 @@ def create_highlighted_versions(output_basepath):
     ):
         try:
             env = os.environ.copy()
-            env.pop("PAGER", None)
-            env.pop("BAT_PAGER", None)
+            env.pop("BAT_CACHE_PATH", None)
+            env.pop("BAT_CONFIG_DIR", None)
             env.pop("BAT_CONFIG_PATH", None)
+            env.pop("BAT_OPTS", None)
+            env.pop("BAT_PAGER", None)
             env.pop("BAT_STYLE", None)
-            env.pop("BAT_THEME", None)
             env.pop("BAT_TABS", None)
+            env.pop("BAT_THEME", None)
+            env.pop("NO_COLOR", None)
+            env.pop("PAGER", None)
             env["COLORTERM"] = "truecolor"  # make sure to output 24bit colors
 
             source_dirname = path.basename(path.dirname(source))