tests: Add test case for environment variables

These additional test cases check if the env section
can:

- export variables with their default value
- add variables with 'None' assigned only to
  BB_ENV_PASSTHROUGH_ADDITIONS or to the deprecated
  BB_ENV_EXTRAWHITE
- bitbake: make sure that variables actually make it into bitbake

The BB_ENV_EXTRAWHITE variable is still present, so a test case has also
been added for this.

Signed-off-by: Benedikt Niedermayr <benedikt.niedermayr@siemens.com>
[Jan: ensure python 3.5 compatibility, adjust copyright header]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_environment_variables/meta-dummy/conf/layer.conf

@@ -0,0 +1,2 @@
+# We have a conf and classes directory, add to BBPATH
+BBPATH .= ":${LAYERDIR}"

tests/test_environment_variables/test_env.yml

@@ -0,0 +1,25 @@
+header:
+  version: 13
+
+env:
+  TESTVAR_DEFAULT_VAL: "BAR"
+  TESTVAR_WHITELIST:
+
+repos:
+  this:
+    layers:
+     meta-dummy:
+
+  # Testing new BB_ENV_PASSTHROUGH_ADDITIONS
+  bitbake_new:
+    url: https://git.openembedded.org/bitbake
+    refspec: 87104b6a167188921da157c7dba45938849fb22a
+    layers:
+      .: excluded
+
+  # Testing deprecated BB_ENV_WHITELIST
+  bitbake_old:
+    url: https://git.openembedded.org/bitbake
+    refspec: efaafc9ec2e8c0475e3fb27e877a1c0a5532a0e5
+    layers:
+      .: excluded