tests: Stress the build_system property

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_build_system/isar-init-build-env

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "isar" > build-env

tests/test_build_system/oe-init-build-env

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+echo "openembedded" > build-env

tests/test_build_system/test-isar.yml

@@ -0,0 +1,7 @@
+header:
+  version: 10
+
+build_system: isar
+
+repos:
+  this:

tests/test_build_system/test-oe.yml

@@ -0,0 +1,7 @@
+header:
+  version: 10
+
+build_system: oe
+
+repos:
+  this:

tests/test_build_system/test-openembedded.yml

@@ -0,0 +1,7 @@
+header:
+  version: 10
+
+build_system: openembedded
+
+repos:
+  this: