Test selfie_hellovar.sh fails if the current timestamp changes while test is executed

[enter-github-username]

While running benchmarks on selFIe, I found that the test selfie_hellovar.sh is flaky, i.e. it fails sometimes.

The test selfie_hellovar.sh saves the current timestamp into the SELFIE_TEST variable. After the code under test finishes, it searches for the timestamp inside the error output of the code under test.

However, and I believe this causes the flaky behaivior, if the timestamp changes between the test itself and the code under test reading the current timestamp (e.g. by the test taking longer than a second or the test being started close to the "end of a second"), the grep call fails, causing the test to fail.