test: Skip integration tests if $HOSTALIASES is not functional

Most integration tests depend on the $HOSTALIASES environment variable
being functional. That variable works on most systems, but not all. In
particular, systems with `systemd-resolved` can cause the variable to be
ignored.

This was reported by Alex Ellwein in
https://github.com/albertito/chasquid/issues/20.

This patch makes the affected tests to be skipped if $HOSTALIASES is not
working properly. It also removes unnecessary hosts files from tests
which don't need it, and documents this behaviour.

Thanks to Alex Ellwein and foxcpp@ for reporting and helping investigate
this issue!

test/README.md

@@ -46,6 +46,12 @@ if the dependencies are not found:
 For some tests, python >= 3.5 is required; they will be skipped if it's not
 available.
 
+Most tests depend on the
+[`$HOSTALIASES`](https://man7.org/linux/man-pages/man7/hostname.7.html)
+environment variable being functional, and will be skipped if it isn't. This
+works by default in most Linux systems, but note that the use of
+`systemd-resolved` can prevent it from working properly.
+
 
 ## Stress tests
 
@@ -55,6 +61,7 @@ load against chasquid to measure performance and resource consumption.
 While they are not exhaustive, they are useful to catch regressions and track
 improvements on the main code paths.
 
+
 ## Fuzz tests
 
 Some Go packages also have instrumentation to run fuzz testing against them,