Disable mem usage test case for non-jemalloc allocators

[zuiderkwast]

Attempt to fix non-jemalloc builds by disabling this test case on non-jemalloc builds:

*** [err]: Memory usage of embedded string value in tests/unit/type/string.tcl
Expected '40' to be less than or equal to '32' (context: type eval line 5 cmd {assert_lessthan_equal [r memory usage quux] 32} proc ::test)

We can't assume anything about allocation size classes for arbitrary allocators.

Glibc malloc seems to give usable sizes of 24, 40, 56, 72, 88, 104, ..., not similar to jemalloc size classes.

Additional change: encode unused robj allocation space in the EMBSTR sds header. This makes the test case able to correctly calculate the memory overhead of the different structures in 32-bit builds, where there is some unused space in the allocation (because the robj header is smaller).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.02%. Comparing base (fc55142) to head (50d3a40).
Report is 3 commits behind head on unstable.

Additional details and impacted files

@@            Coverage Diff            @@
##           unstable    #1685   +/-   ##
=========================================
  Coverage     71.02%   71.02%           
=========================================
  Files           121      121           
  Lines         65254    65255    +1     
=========================================
+ Hits          46344    46347    +3     
+ Misses        18910    18908    -2     

Files with missing lines	Coverage Δ
src/object.c	82.19% <100.00%> (+0.01%)	⬆️

... and 11 files with indirect coverage changes