test: add --no-single-source to check whether normal build process works

code/scripts/pio_pre.py

@@ -150,7 +150,7 @@ if len(ino) == 1 and ino[0].name == "espurna.ino":
     env.AddMethod(ConvertInoToCpp)
 
 # merge every .cpp into a single file and **only** build that single file
-if os.environ.get("ESPURNA_BUILD_SINGLE_SOURCE"):
+if "1" == os.environ.get("ESPURNA_BUILD_SINGLE_SOURCE", "0"):
     cpp_files = []
     for root, dirs, filenames in os.walk("espurna"):
         for name in filenames:

code/scripts/test_build.py

@@ -80,7 +80,8 @@ def run_configurations(args, configurations):
         os_env = os.environ.copy()
         os_env["PLATFORMIO_SRC_BUILD_FLAGS"] = "-DUSE_CUSTOM_H"
         os_env["PLATFORMIO_BUILD_CACHE_DIR"] = "test/pio_cache"
-        os_env["ESPURNA_BUILD_SINGLE_SOURCE"] = "1"
+        if not args.no_single_source:
+            os_env["ESPURNA_BUILD_SINGLE_SOURCE"] = "1"
 
         start = time.time()
         subprocess.check_call(cmd, env=os_env)
@@ -171,5 +172,8 @@ if __name__ == "__main__":
     parser.add_argument(
         "--no-silent", action="store_true", help="Do not silence pio-run"
     )
+    parser.add_argument(
+        "--no-single-source", action="store_true", help="Disable 'unity' build"
+    )
 
     main(parser.parse_args())