Fix some characterization tests

A few changes had cropped up with `_NIX_TEST_ACCEPT=1`:

1. Blake hashing test JSON had a different indentation

2. Store URI had improper non-quoted spaces

(1) was is just fixed, as we trust nlohmann JSON to parse JSON
correctly, regardless of whitespace.

For (2), the existing URL was made a read-only test, since we very much
wish to continue parsing such invalid URLs directly. And then the
original read/write test was updated to properly percent-encode the
space, as the normal form should be.

src/libstore-tests/data/store-reference/local_3.txt

@@ -1 +1 @@
-local://?root=/foo bar/baz
+local://?root=/foo%20bar/baz

src/libstore-tests/data/store-reference/local_3_no_percent.txt

@@ -0,0 +1 @@
+local://?root=/foo bar/baz