os: add os.quoted_path/1, use it consistently for running V itself

vlib/os/os.v

@@ -745,3 +745,12 @@ pub fn execute_or_exit(cmd string) Result {
 	}
 	return res
 }
+
+// quoted path - return a quoted version of the path, depending on the platform.
+pub fn quoted_path(path string) string {
+	$if windows {
+		return '"$path"'
+	} $else {
+		return "'$path'"
+	}
+}

vlib/os/os_test.v

@@ -851,7 +851,7 @@ fn test_execute() ? {
 	defer {
 		os.rm(print0script) or {}
 	}
-	result := os.execute('"' + @VEXE + '" run "$print0script"')
+	result := os.execute('${os.quoted_path(@VEXE)} run ${os.quoted_path(print0script)}')
 	hexresult := result.output.bytes().hex()
 	// println('exit_code: $result.exit_code')
 	// println('output: |$result.output|')

vlib/os/process_test.v

@@ -18,7 +18,7 @@ fn testsuite_begin() ? {
 		//   WINE_TEST_OS_PROCESS_EXE=x.exe ./v -os windows vlib/os/process_test.v
 		os.cp(os.getenv('WINE_TEST_OS_PROCESS_EXE'), test_os_process) ?
 	} else {
-		os.system('$vexe -o $test_os_process $test_os_process_source')
+		os.system('${os.quoted_path(vexe)} -o ${os.quoted_path(test_os_process)} ${os.quoted_path(test_os_process_source)}')
 	}
 	assert os.exists(test_os_process)
 }