tests: Document some relevant environment variables

Signed-off-by: Simon McVittie <[email protected]>

tests/README.md

@@ -0,0 +1,46 @@
+xdg-desktop-portal test suite
+=============================
+
+## Environment
+
+Some relevant environment variables that can be set during testing,
+but should not normally be set on production systems:
+
+* `FLATPAK_BWRAP`: Path to the **bwrap**(1) executable
+    (default: discovered at build-time)
+
+* `LIBEXECDIR`: If set, look for the x-d-p executable in this directory
+
+* `TEST_IN_CI`: If set (to any value), some tests that are not always
+    reliable are skipped.
+    Set this for automated QA testing, leave it unset during development.
+
+* `XDP_VALIDATE_ICON_INSECURE`: If set (to any value), x-d-p doesn't
+    sandbox the icon validator using **bwrap**(1), even if sandboxed
+    validation was enabled at compile time.
+    This can be used to run build-time tests in a chroot or unprivileged
+    container environment, where **bwrap**(1) normally can't work.
+    It should never be set on a production system that will be validating
+    untrusted icons!
+
+* `XDP_VALIDATE_SOUND_INSECURE`: Same as `XDP_VALIDATE_ICON_INSECURE`,
+    but for sounds
+
+### Used automatically
+
+These environment variables are set automatically and shouldn't need to be
+changed, but developers improving the test suite might need to be aware
+of them:
+
+* `XDG_DESKTOP_PORTAL_DIR`: If set, it will be used instead of the
+    compile-time path (normally `/usr/share/xdg-desktop-portal/portals`)
+
+* `XDP_UNINSTALLED`: Set to 1 when running build-time tests on a version
+    of x-d-p that has not yet been installed. Leave unset when running
+    "as-installed" tests on the system copy of x-d-p.
+
+* `XDP_VALIDATE_ICON`: Path to `x-d-p-validate-icon` executable in the
+    build directory
+
+* `XDP_VALIDATE_SOUND`: Path to `x-d-p-validate-sound` executable in the
+    build directory