pw_string: Add tests missing from test group
pw_string: Add tests missing from test group

Change-Id: Ifbe35c8e5c32014ffe30a7755482a7e3906414ce

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed