Test suite depends on submodule instead of using gtest from distribution

Looks like some files are missing in dist tar ball.

[tkloczko@barrel tesseract-4.1.1]$ make check
Making check in src/arch
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Making check in src/viewer
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Making check in src/wordrec
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Making check in src/textord
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Making check in src/api
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Making check in .
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Making check in tessdata
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Making check in configs
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Making check in doc
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Making check in unittest
make[1]: Entering directory '/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1/unittest'
make  apiexample_test applybox_test baseapi_test  bitvector_test cleanapi_test colpartition_test commandlineflags_test dawg_test denorm_test  fileio_test heap_test imagedata_test indexmapbidi_test intfeaturemap_test intsimdmatrix_test lang_model_test layout_test ligature_table_test linlsq_test lstm_recode_test lstm_squashed_test lstm_test lstmtrainer_test loadlang_test mastertrainer_test matrix_test  normstrngs_test nthitem_test osd_test pagesegmode_test  paragraphs_test params_model_test progress_test qrsequence_test recodebeam_test rect_test resultiterator_test scanutils_test shapetable_test stats_test  stringrenderer_test tablefind_test tablerecog_test tabvector_test  textlineprojection_test tfile_test unichar_test unicharcompress_test unicharset_test validate_grapheme_test validate_indic_test validate_khmer_test validate_myanmar_test validator_test libgtest.la libgtest_main.la libgmock.la libgmock_main.la libabseil.la
make[2]: Entering directory '/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1/unittest'
g++ -DHAVE_CONFIG_H -I. -I..  -O2 -DNDEBUG -DTESSBIN_DIR="\"/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1\"" -DLANGDATA_DIR="\"/home/tkloczko/rpmbuild/BUILD/langdata_lstm\"" -DTESSDATA_DIR="\"/home/tkloczko/rpmbuild/BUILD/tessdata\"" -DTESSDATA_BEST_DIR="\"/home/tkloczko/rpmbuild/BUILD/tessdata_best\"" -DTESTING_DIR="\"/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1/test/testing\"" -DTESTDATA_DIR="\"/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1/test/testdata\"" -DPANGO_ENABLE_ENGINE  -I../src/api -I../src/api -I../src/arch -I../src/ccmain -I../src/ccstruct -I../src/ccutil -I../src/classify -I../src/cutil -I../src/dict -I../src/display -I../src/lstm -I../src/textord -I../unittest/base -I../unittest/util -I../src/training -I../src/viewer -I../src/wordrec -I../abseil  -isystem ../googletest/googletest/include -isystem ../googletest/googlemock/include -I/usr/include/leptonica     -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/harfbuzz  -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng16   -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fstack-protector-strong -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -flto=auto -flto-partition=none -std=c++17 -c -o apiexample_test.o apiexample_test.cc
make[2]: *** No rule to make target '../googletest/googletest/src/gtest-all.cc', needed by '../googletest/googletest/src/libgtest_la-gtest-all.lo'.  Stop.
make[2]: Leaving directory '/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1/unittest'
make[1]: *** [Makefile:2656: check-am] Error 2
make[1]: Leaving directory '/home/tkloczko/rpmbuild/BUILD/tesseract-4.1.1/unittest'
make: *** [Makefile:502: check-recursive] Error 1