cmake: Make building of tests optional

[GBuella]

Introduce the BUILD_LIBPMEMFILE_POSIX_TESTS and BUILD_LIBPMEMFILE_TESTS options.

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This change is 

[codecov[bot]]

Codecov Report

Merging #339 into master will increase coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
+ Coverage   73.66%   73.71%   +0.05%     
==========================================
  Files          64       64              
  Lines        7990     7990              
  Branches     1613     1613              
==========================================
+ Hits         5886     5890       +4     
+ Misses       1588     1586       -2     
+ Partials      516      514       -2

Flag	Coverage Δ
#sqlite_tests	45.55% <ø> (+0.01%)	:arrow_up:
#tests_antool	12.21% <ø> (ø)	:arrow_up:
#tests_posix_multi_threaded	26.04% <ø> (+0.15%)	:arrow_up:
#tests_posix_single_threaded	56.04% <ø> (-0.03%)	:arrow_down:
#tests_preload	45.43% <ø> (-0.01%)	:arrow_down:

Impacted Files	Coverage Δ
src/libpmemfile-posix/dir.c	85.5% <0%> (+0.49%)	:arrow_up:
src/libpmemfile-posix/rename.c	92% <0%> (+0.79%)	:arrow_up:

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[krzycz]

Reviewed 2 of 2 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[krzycz]

Reviewed 1 of 2 files at r2, 1 of 1 files at r3. Review status: all files reviewed at latest revision, 3 unresolved discussions.

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Comments from Reviewable

[marcinslusarz]

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Reviewed 1 of 2 files at r2, 1 of 1 files at r3. Review status: :shipit: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[sarahjelinek]

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Review status: :shipit: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable