Use gtest: Add initial test

[smhmhmd]

More tests need to be added in test directory.
Code-coverage info is in the web pages in the CODE_COVERAGE directory.

Instructions:
cmake ../ -DCODE_COVERAGE=ON
make -j
./credentials-fetcher-test
cd CMakeFiles/credentials-fetcher-test.dir/test
ls *.gcno
gcov tester.cpp.gcno
lcov --capture --directory . --output-file gtest_coverage.info --ignore-errors mismatch
genhtml gtest_coverage.info --output-directory CODE_COVERAGE

Logs:
$ ./credentials-fetcher-test
Running main() from /builddir/build/BUILD/googletest-release-1.11.0/googletest/src/gtest_main.cc
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from DaemonTest
[ RUN ] DaemonTest.InvalidCharacterTest
[ OK ] DaemonTest.InvalidCharacterTest (0 ms)
[----------] 1 test from DaemonTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 1 test.

$  genhtml gtest_coverage.info --output-directory CODE_COVERAGE
Found 17 entries.
Found common filename prefix "/usr/include/c++"
Generating output.
Processing file 11/bits/move.h
  lines=2 hit=0 functions=3 hit=0
Processing file /usr/include/gtest/internal/gtest-internal.h
  lines=19 hit=17 functions=8 hit=7
Processing file 11/bits/unique_ptr.h
  lines=18 hit=6 functions=13 hit=2
Processing file /home/ec2-user/credentials-fetcher/test/tester.cpp
  lines=2 hit=2 functions=4 hit=4
Processing file 11/tuple
  lines=14 hit=6 functions=20 hit=4
Processing file 11/bits/stl_iterator_base_funcs.h
  lines=5 hit=0 functions=4 hit=0
Processing file 11/bits/alloc_traits.h
  lines=2 hit=0 functions=1 hit=0
Processing file 11/bits/stl_iterator_base_types.h
  lines=2 hit=0 functions=2 hit=0
Processing file 11/bits/char_traits.h
  lines=12 hit=0 functions=3 hit=0
Processing file /usr/include/gtest/gtest.h
  lines=17 hit=6 functions=11 hit=3
Processing file 11/bits/basic_string.tcc
  lines=13 hit=0 functions=2 hit=0
Processing file 11/ext/new_allocator.h
  lines=3 hit=0 functions=3 hit=0
Processing file /usr/include/gtest/gtest-printers.h
  lines=30 hit=0 functions=16 hit=0
Processing file 11/bits/basic_string.h
  lines=11 hit=0 functions=3 hit=0
Processing file 11/ext/alloc_traits.h
  lines=2 hit=0 functions=1 hit=0
Processing file /usr/include/gtest/internal/gtest-port.h
  lines=1 hit=0 functions=1 hit=0
Processing file 11/ext/type_traits.h
  lines=2 hit=0 functions=2 hit=0
Overall coverage rate:
  lines......: 23.9% (37 of 155 lines)
  functions......: 20.6% (20 of 97 functions)

$ ls CODE_COVERAGE/
11         cmd_line     gcov.css   home               index-sort-l.html  ruby.png  updown.png
amber.png  emerald.png  glass.png  index-sort-f.html  index.html         snow.png  usr

Issue #, if available:

Description of changes:

Testing done:

Merge Checklist

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General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and comments (where appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific environment

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