osutil: honor SNAPD_UNSAFE_IO for testing

[zyga]

When SNAPD_UNSAFE_IO is set and we are testing (the executable ends with
.test) then skip some sync calls that are very very heavy. Doing this
can easily cut the timy by an order of magnitude.

As a non-scientific but useful measurement, overlord tests go down from
2 minutes 41 seconds to just 15 seconds. This is not representative to
all modules but when it matters, it is noticeable.

zyga@faroe:~/snapd/overlord> time SNAPD_UNSAFE_IO=0 go test
OK: 41 passed
PASS
ok  	github.com/snapcore/snapd/overlord	156.342s

real	2m41,765s
user	0m12,964s
sys	0m7,061s

zyga@faroe:~/snapd/overlord> time SNAPD_UNSAFE_IO=1 go test
OK: 41 passed
PASS
ok  	github.com/snapcore/snapd/overlord	9.963s

real	0m15,380s
user	0m12,217s
sys	0m1,626s

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[codecov-io]

Codecov Report

Merging #3725 into master will increase coverage by 0.13%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #3725      +/-   ##
==========================================
+ Coverage   75.84%   75.98%   +0.13%     
==========================================
  Files         399      399              
  Lines       34525    34617      +92     
==========================================
+ Hits        26184    26302     +118     
+ Misses       6478     6460      -18     
+ Partials     1863     1855       -8

Impacted Files	Coverage Δ
osutil/io.go	69.23% <50%> (+0.48%)	⬆️
overlord/ifacestate/helpers.go	63% <0%> (+0.66%)	⬆️
interfaces/sorting.go	100% <0%> (+1.28%)	⬆️
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️
asserts/assertstest/assertstest.go	71.61% <0%> (+2.05%)	⬆️
cmd/snap-repair/runner.go	92.42% <0%> (+9.46%)	⬆️

---

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[niemeyer]

LGTM, but let's please make the variable more clear:

[niemeyer]

Let's please rename this to snapdUnsafeIO so the relationship to the variable is clear (we need to invert the tests too). We can also drop the = true/false part as it defaults to false.

In fact, we can probably inline:

var snapdUsafeIO = len(os.Args) > 0 && strings.HasSuffix(os.Args[0], ".test") && GetenvBool("SNAPD_UNSAFE_IO")

[zyga]

Done

[mvo5]

Looks great! One suggestion/nitpick but feel free to ignore and merge anyway.

[mvo5]

(nitpick) - I would declare this at the top of the file somewhere

[zyga]

Done