target: tests: Address review comments on PR#667

PR#667: https://github.com/ARM-software/devlib/pull/667

- Implement a test module initializer with a tear down method in
  test/test_target.py
- Make various cleanups in test/test_target.py
- Improve structure of test/test_config.yml (previously
  target_configs.yaml)
- Make docstrings Sphinx compatible
- Make ``TargetRunner`` and its subclasses private
- Cleanup tests/test_target.py
- Replace print()'s with appropriate logging calls
- Implement ``NOPTargetRunner`` class for simplifying tests
- Improve Python v3.7 compatibility
- Relax host machine type checking
- Escape user input strings

and more..

Signed-off-by: Metin Kaya <metin.kaya@arm.com>

tests/test_config.yml.example

@@ -0,0 +1,40 @@
+target-configs:
+  entry-0:
+    AndroidTarget:
+      timeout: 60
+      connection_settings:
+        device: 'emulator-5554'
+
+  entry-1:
+    ChromeOsTarget:
+      connection_settings:
+        device: 'emulator-5556'
+        host: 'example.com'
+        username: 'username'
+        password: 'password'
+
+  entry-2:
+    LinuxTarget:
+      connection_settings:
+        host: 'example.com'
+        username: 'username'
+        password: 'password'
+
+  entry-3:
+    LocalLinuxTarget:
+      connection_settings:
+        unrooted: True
+
+  entry-4:
+    QEMUTargetRunner:
+        qemu_settings:
+          kernel_image: '/path/to/devlib/tools/buildroot/buildroot-v2023.11.1-aarch64/output/images/Image'
+
+  entry-5:
+    QEMUTargetRunner:
+      connection_settings:
+        port: 8023
+      qemu_settings:
+        kernel_image: '/path/to/devlib/tools/buildroot/buildroot-v2023.11.1-x86_64/output/images/bzImage'
+        arch: 'x86_64'
+        cmdline: 'console=ttyS0'