intel/compiler tests: rewrite subprocess handling in run-test.py

`subprocess.Popen()` returns immediately, and the subprocess might not
have finished by the time `stdout` is read on the next line, spuriously
failing the tests.

`subprocess.check_output()` makes sure the output is available before
returning, solving this issue; it additionally raises an error if the
subprocess failed, giving a better error than a failed diff later in the
script.

cc: mesa-stable

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34756>

src/intel/compiler/elk/tests/run-test.py

@@ -48,10 +48,8 @@ for asm_file in args.gen_folder.glob('*.asm'):
             '--gen', args.gen_name,
             asm_file
         ]
-        with subprocess.Popen(command,
-                              stdout=subprocess.PIPE,
-                              stderr=subprocess.DEVNULL) as cmd:
-            lines_after = [line.decode('ascii') for line in cmd.stdout.readlines()]
+        stdout = subprocess.check_output(command, timeout=1).decode()
+        lines_after = stdout.splitlines(keepends=True)
     except OSError as e:
         if e.errno == errno.ENOEXEC:
             print('Skipping due to inability to run host binaries.',

src/intel/compiler/tests/run-test.py

@@ -48,10 +48,8 @@ for asm_file in args.gen_folder.glob('*.asm'):
             '--gen', args.gen_name,
             asm_file
         ]
-        with subprocess.Popen(command,
-                              stdout=subprocess.PIPE,
-                              stderr=subprocess.DEVNULL) as cmd:
-            lines_after = [line.decode('ascii') for line in cmd.stdout.readlines()]
+        stdout = subprocess.check_output(command, timeout=1).decode()
+        lines_after = stdout.splitlines(keepends=True)
     except OSError as e:
         if e.errno == errno.ENOEXEC:
             print('Skipping due to inability to run host binaries.',