make test fails in PHP 7.4

[garbetjie]

When installing the opencensus-php extension from source, running make test fails on tests/name_uncastable_object.phpt.

The expected output is:

%s fatal error: Object of class UncastableObject could not be converted to string in %s/name_uncastable_object.php on line %d

The actual output received is:

Fatal error: Uncaught Error: Object of class UncastableObject could not be converted to string in /tmp/opencensus-php-d1512abf456761165419a7b236e046a38b61219e/ext/tests/name_uncastable_object.php:9
Stack trace:
#0 /tmp/opencensus-php-d1512abf456761165419a7b236e046a38b61219e/ext/tests/name_uncastable_object.php(9): opencensus_trace_begin('foo', Array)
#1 {main}
  thrown in /tmp/opencensus-php-d1512abf456761165419a7b236e046a38b61219e/ext/tests/name_uncastable_object.php on line 9