Rework fseek/ftell tests
Rework fseek/ftell tests

ARM C library is really good at optimising out calls to underlying
seek. The only ones we were actually detecting in the empty file case
were the ones that the default FileHandle::size() made itself during
the SEEK_END case.

When we implement TestFile::size() directly, we will no longer see
a single seek call from the ARM C library in the empty file case, so
remove those tests.

Beef up the non-empty file case, adding checks that we are making
underlying read+write calls in the correct position, as a proxy for
direct checks for underlying seek being called.

kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed