Linux/ARM: Add execution time of ./tests/runtest.sh

Linux/ARM: Add execution time of ./tests/runtest.sh

We have to wait for a long time to complete all unit tests on the ARM-based
embedded boards (e.g, Raspberry Pi, Odroid, Chromebook) compared to the
X86-based server environment. So, let's display execution time after finishing
the unit test.

* After PR:
128 minutes and 36 seconds taken to run CoreCLR tests.

ver1:
The existing measured time still includes overlay creation and some
package restoration. So, let's put time_start in front of
if($testDirectories).

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Reviewed-by: Aditya Mandaleeka <adityamandaleeka@microsoft.com>
Reviewed-by: Sergiy Kuryata <sergiy-k@microsoft.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/4f330fdcb0bfee6372584b28a098a14bc7803c5c