Add test using vfork

[GBuella]

This change is 

Ref: #3

[codecov-io]

Codecov Report

Merging #50 into master will increase coverage by 0.57%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   69.71%   70.28%   +0.57%     
==========================================
  Files          19       20       +1     
  Lines        1433     1454      +21     
  Branches      391      397       +6     
==========================================
+ Hits          999     1022      +23     
+ Misses        235      234       -1     
+ Partials      199      198       -1

Flag	Coverage Δ
#pmemfile_tests	65.67% <ø> (ø)	⬆️
#regular_tests	57.15% <71.42%> (+0.76%)	⬆️

Impacted Files	Coverage Δ
test/prog_using_vfork.c	71.42% <71.42%> (ø)
src/intercept_util.c	54.65% <0%> (+1.05%)	⬆️
src/intercept.c	68.05% <0%> (+1.38%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e15d384...1743bd8. Read the comment docs.

[sarahjelinek]

---

Review status: 0 of 3 files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[krzycz]

---

Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable